Expanded Contributing Guidelines (Kotlin#1076)

* Expanded Contributing Guidelines

* Provide more context to contributors to avoid most common mistakes.
* Mention API stability rules.
* Provide instructions on using knit and apiDump tools.
* Mention preferred ways of contacting maintainers.

Author: elizarov

CONTRIBUTING.md

@@ -1,33 +1,99 @@
-# Contributing
+# Contributing Guidelines
 
-## Report a bug
+There are two main ways to contribute to the project — submitting issues and submitting 
+fixes/changes/improvements via pull requests.
 
-Just use a 'bug report' template when creating a new issue.
+## Submitting issues
 
-## Submit a PR
+Both bug reports and feature requests are welcome.
+Submit issues [here](https://github.com/Kotlin/kotlinx.serialization/issues).
 
-The process of submitting a pull request depends on what kind of changes you have in it:
+* Search for existing issues to avoid reporting duplicates.
+* When submitting a bug report:
+  * Use a 'bug report' template when creating a new issue.
+  * Test it against the most recently released version. It might have been already fixed.
+  * By default, we assume that your problem reproduces in Kotlin/JVM. Please, mention if the problem is
+    specific to Kotlin/JS or Kotlin/Native. 
+  * Include the code that reproduces the problem. Provide the complete reproducer code, yet minimize it as much as possible.
+  * However, don't put off reporting any weird or rarely appearing issues just because you cannot consistently 
+    reproduce them.
+  * If the bug is in behavior, then explain what behavior you've expected and what you've got.  
+* When submitting a feature request:
+  * Use a 'feature request' template when creating a new issue.
+  * Explain why you need the feature — what's your use-case, what's your domain.
+  * Explaining the problem you face is more important than suggesting a solution. 
+    Report your problem even if you don't have any proposed solution.
+  * If there is an alternative way to do what you need, then show the code of the alternative.
 
-#### Fix typo/documentation clarification
+## Submitting PRs
 
-You can open your PR directly against a `master` branch.
+We love PRs. Submit PRs [here](https://github.com/Kotlin/kotlinx.serialization/pulls).
+However, please keep in mind that maintainers will have to support the resulting code of the project,
+so do familiarize yourself with the following guidelines. 
 
-#### Fix a minor bug
+* All development (both new features and bug fixes) is performed in the `dev` branch.
+  * The `master` branch always contains sources of the most recently released version.
+  * Base PRs against the `dev` branch.
+  * The `dev` branch is pushed to the `master` branch during release.
+  * Documentation in markdown files can be updated directly in the `master` branch, 
+    unless the documentation is in the source code, and the patch changes line numbers.
+* If you fix documentation:
+  * After fixing/changing code examples in the [`docs`](docs) folder or updating any references in the markdown files
+    run the [Knit tool](#running-the-knit-tool) and commit the resulting changes as well. 
+    It will not pass the tests otherwise.
+  * If you plan extensive rewrites/additions to the docs, then please [contact the maintainers](#contacting-maintainers)
+    to coordinate the work in advance.    
+* If you make any code changes:
+  * Follow the [Kotlin Coding Conventions](https://kotlinlang.org/docs/reference/coding-conventions.html). 
+    * Use 4 spaces for indentation. 
+    * Use imports with '*'.
+  * [Build the project](#building) to make sure it all works and passes the tests.
+* If you fix a bug:
+  * Write the test the reproduces the bug.
+  * Fixes without tests are accepted only in exceptional circumstances if it can be shown that writing the 
+    corresponding test is too hard or otherwise impractical.
+  * Follow the style of writing tests that is used in this project: 
+    name test functions as `testXxx`. Don't use backticks in test names.
+* If you introduce any new public APIs:
+  * All new APIs must come with documentation and tests.
+  * All new APIs are initially released with `@ExperimentalSerializationApi` annotation and are graduated later.
+  * [Update the public API dumps](#updating-the-public-api-dump) and commit the resulting changes as well. 
+    It will not pass the tests otherwise.
+  * If you plan large API additions, then please start by submitting an issue with the proposed API design  
+    to gather community feedback.
+  * [Contact the maintainers](#contacting-maintainers) to coordinate any big piece of work in advance.
+* If you propose/implement a new serialization format:
+  * Follow the general advice on new public APIs above.
+  * Note, that you can keep new format implementation in your own repository to be able to perform proper maintenance 
+    and to have a separate release cycle. 
+  * You can submit a PR to the [list of community-supported formats](formats/README.md#other-community-supported-formats) 
+    with a description of your library.
+* Comment on the existing issue if you want to work on it. Ensure that the issue not only describes a problem,
+  but also describes a solution that had received a positive feedback. Propose a solution if there isn't any.
 
-Open your PR against the `dev` branch of the repository with a meaningful message in the description.
+## Building
 
-#### Fix a bug in the library core/implement a new feature
+You can find all the instructions [here](docs/building.md)
 
-First, open an issue from 'feature request' template or describe in the existing issue what idea you are trying to implement
-– probably it needs some tuning to be more aligned with the whole library design and vision.
-After a design discussion, you can open a PR against the `dev` branch.
+### Running the Knit tool
 
-## Propose/implement a new serialization format
+* Use [Knit](https://github.com/Kotlin/kotlinx-knit/blob/master/README.md) for updates to documentation:
+  * Run `./gradlew knit` to update example files, links, tables of content.
+  * Commit updated documents and examples together with other changes.
 
-If you have created a new serialization format, you can keep it
-in your repository to be able to perform proper maintenance and to have a separate release cycle. 
-You can submit a PR to the [list of community-supported formats](formats/README.md#other-community-supported-formats) with a description of your library.
+### Updating the public API dump
 
-## Building this library
+* Use [Binary Compatibility Validator](https://github.com/Kotlin/binary-compatibility-validator/blob/master/README.md) for updates to public API:
+  * Run `./gradlew apiDump` to update API index files. 
+  * Commit updated API indexes together with other changes.
+
+## Releases
+
+* Full release procedure checklist is [here](RELEASING.md).
+
+## Contacting maintainers
+
+* If something cannot be done, not convenient, or does not work — submit an [issue](#submitting-issues).
+* "How to do something" questions — [StackOverflow](https://stackoverflow.com).
+* Discussions and general inquiries — use `#serialization` channel in [KotlinLang Slack](https://kotl.in/slack).
 
-You can find all the instructions [here](docs/building.md)