[#2064] Add CONTRIBUTING.md

Author: DavideD

CONTRIBUTING.md

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+# Contributing
+
+Contributions from the community are essential in keeping Hibernate (and any Open Source
+project really) strong and successful.  
+
+# Legal
+
+All original contributions to Hibernate are licensed under the 
+[Apache 2.0 license](https://www.apache.org/licenses/LICENSE-2.0).
+
+The Apache 2.0 license text is included verbatim in the [LICENSE](LICENSE) file in the root directory
+of the Hibernate Reactive repository.
+
+All contributions are subject to the [Developer Certificate of Origin (DCO)](https://developercertificate.org/).  
+
+The DCO text is available verbatim in the [dco.txt](dco.txt) file in the root directory
+of the Hibernate Reactive repository.
+
+## Guidelines
+
+While we try to keep requirements for contributing to a minimum, there are a few guidelines 
+we ask that you mind.
+
+For code contributions, these guidelines include:
+* Respect the project code style - find templates for [IntelliJ IDEA](https://hibernate.org/community/contribute/intellij-idea/) or [Eclipse](https://hibernate.org/community/contribute/eclipse-ide/)
+* Have a corresponding GitHub [issue](https://github.com/hibernate/hibernate-reactive/issues) and be sure to include
+  the key for this issue in your commit messages.
+* Have a set of appropriate tests.  
+  For your convenience, a [set of test templates](https://github.com/hibernate/hibernate-test-case-templates/tree/main/reactive)
+  have been made available.
+  	
+  When submitting bug reports, the tests should reproduce the initially reported bug and illustrate that your solution addresses the issue.
+  For features/enhancements, the tests should demonstrate that the feature works as intended.  
+  In both cases, be sure to incorporate your tests into the project to protect against possible regressions.
+* If applicable, documentation should be updated to reflect the introduced changes
+* The code compiles and the tests pass (`./gradlew clean build`)
+
+For documentation contributions, mainly to respect the project code style, especially in regard 
+to the use of tabs - as mentioned above, code style templates are available for both IntelliJ IDEA and Eclipse
+IDEs.  Ideally, these contributions would also have a corresponding issue, although this 
+is less necessary for documentation contributions.
+
+## Getting Started
+
+If you are just getting started with Git, GitHub, and/or contributing to Hibernate via
+GitHub there are a few pre-requisite steps to follow:
+
+* Make sure you have a [GitHub account](https://github.com/signup/free)
+* [Fork](https://help.github.com/articles/fork-a-repo) the Hibernate Reactive repository.  As discussed in
+the linked page, this also includes:
+    * [set up your local git install](https://help.github.com/articles/set-up-git) 
+    * clone your fork
+* Instruct git to ignore certain commits when using `git blame`. From the directory of your local clone, run this: `git config blame.ignoreRevsFile .git-blame-ignore-revs`
+* See the wiki pages for setting up your IDE, whether you use 
+[IntelliJ IDEA](https://hibernate.org/community/contribute/intellij-idea/)
+or [Eclipse](https://hibernate.org/community/contribute/eclipse-ide/)<sup>(1)</sup>.
+
+
+## Create the working (topic) branch
+
+Create a [topic branch](https://git-scm.com/book/en/Git-Branching-Branching-Workflows#Topic-Branches) 
+on which you will work.  The convention is to incorporate the JIRA issue key in the name of this branch,
+although this is more of a mnemonic strategy than a hard-and-fast rule - but doing so helps:
+* Remember what each branch is for 
+* Isolate the work from other contributions you may be working on
+
+_If there is not already a GitHub issue covering the work you want to do, [create one](https://github.com/hibernate/hibernate-reactive/issues/new)._
+  
+Assuming you will be working from the `main` branch and working
+on the GitHub issue #123 : `git checkout -b 123 main`
+
+## Code
+
+Do your thing!
+
+
+## Commit
+
+* Make commits of logical units
+* Be sure to start each commit message using the ** GitHub issue key **. For example:
+  ```
+  [#1234] Fix some kind of problem
+  ```
+* Make sure you have added the necessary tests for your changes
+* Run _all_ the tests to ensure nothing else was accidentally broken
+
+_Before committing, if you want to pull in the latest upstream changes (highly
+appreciated btw), please use rebasing rather than merging.  Merging creates
+"merge commits" that invariably muck up the project timeline._
+
+## Submit
+
+* Push your changes to the topic branch in your fork of the repository
+* Initiate a [pull request](https://help.github.com/articles/creating-a-pull-request)
+* Adding the sentence `Fix #123`, where `#123` is the issue key, will link the pull request to the corresponding issue
+  and close it accordingly, when the pull request gets merged.
+
+It is important that this topic branch of your fork:
+
+* Is isolated to just the work on this one issue, or multiple issues if they are
+  related and also fixed/implemented by this work.  The main point is to not push commits for more than
+  one PR to a single branch - GitHub PRs are linked to a branch rather than specific commits
+* remain until the PR is closed. Once the underlying branch is deleted the corresponding PR will be closed,
+  if not already, and the changes will be lost.
+
+# Notes
+<sup>(1)</sup> Gradle `eclipse` plugin is no longer supported, so the recommended way to import the project in your IDE is with the proper IDE tools/plugins. Don't try to run `./gradlew clean eclipse --refresh-dependencies` from the command line as you'll get an error because `eclipse` no longer exists