Move contribution guidelines into CONTRIBUTING page (#112)

* Move contribution guidelines into CONTRIBUTING page

Minor improvements to phrasing in the plugin documentation.  Include
instructions for developers that want to use the plugin, modeled after
the HTTP client API 4 instructions.

* Remove unmaintained GitPod references

Author: MarkEWaite

.gitpod.Dockerfile

@@ -0,0 +1,59 @@
+# Contributing to the plugin
+
+Plugin source code is hosted on [GitHub](https://github.com/jenkinsci/docker-java-api-plugin).
+New feature proposals and bug fix proposals should be submitted as
+[GitHub pull requests](https://help.github.com/articles/creating-a-pull-request).
+Your pull request will be evaluated by the [Jenkins job](https://ci.jenkins.io/job/Plugins/job/docker-java-api-plugin/).
+
+Before submitting your change, please assure that you've added tests which verify your change.
+
+## Code formatting with spotless
+
+Source code and pom file formatting are maintained by the `spotless` maven plugin.
+Before submitting a pull request, please format with:
+
+* `mvn spotless:apply`
+
+## Static analysis with spotbugs
+
+Please don't introduce new spotbugs output.
+
+* `mvn spotbugs:check` to analyze project using [Spotbugs](https://spotbugs.github.io)
+* `mvn spotbugs:gui` to review report using GUI
+
+## Code Coverage
+
+[JaCoCo code coverage](https://www.jacoco.org/jacoco/) reporting is available as a maven target and is [reported](https://ci.jenkins.io/job/Plugins/job/docker-java-api-plugin/) by the [Jenkins warnings plugin](https://plugins.jenkins.io/warnings-ng/).
+Please try to improve code coverage with tests when you submit a pull request.
+
+* `mvn -P enable-jacoco clean install jacoco:report` to report code coverage with JaCoCo.
+
+### Reviewing Code Coverage
+
+The code coverage report is a set of HTML files that show methods and lines executed.
+The following commands will open the `index.html` file in the browser.
+
+* Windows - `start target\site\jacoco\index.html`
+* Linux - `xdg-open target/site/jacoco/index.html`
+
+The file will have a list of package names.
+Click on them to find a list of class names.
+
+The lines of the code will be covered in three different colors, red, green, and orange.
+Red lines are not covered in the tests.
+Green lines are covered with tests.
+
+## Maintaining automated tests
+
+Automated tests are run as part of the `verify` phase.
+Run automated tests in a development environment with the command:
+
+```
+$ mvn clean verify
+```
+
+## Report an Issue
+
+Use the ["Report an issue" page](https://www.jenkins.io/participate/report-issue/redirect/#23136) to submit bug reports.
+Please review [existing issues](https://issues.jenkins.io/issues/?jql=resolution%20is%20EMPTY%20and%20component%3D23136) before submitting a new issue.
+Please use the ["How to Report an Issue"](https://www.jenkins.io/participate/report-issue/) guidelines when reporting issues.

.gitpod.yml

@@ -1,32 +1,35 @@
 # Docker Java API Plugin
 
-This plugin exposes the [docker-java](http://github.com/docker-java/docker-java) API to Jenkins plugins.
+This plugin provides the [docker-java](http://github.com/docker-java/docker-java) API to Jenkins plugins.
 Plugins using docker-java should depend on this plugin and not directly on docker-java.
 
 Only the Apache HttpClient 5 transport is available; the Jersey transport does not work.
 
-# Environment
-
-The following build environment is required to build this plugin
-
-* Java 11 and Maven 3.8.1
-
-# Build
-
-To build the plugin locally:
-
-    mvn clean verify
-
-# Release
-
-To release the plugin:
-
-    mvn release:prepare release:perform -B
-
-# Test local instance
-
-To test in a local Jenkins instance
-
-    mvn hpi:run
-
-  [wiki]: http://wiki.jenkins-ci.org/display/JENKINS/Docker+Java+API+Plugin
+## Using the API in your plugin
+
+Replace the dependency to `com.github.docker-java:docker-java` with a dependency to `org.jenkins-ci.plugins:docker-java-api`.
+Avoid version conflicts by using the [Jenkins plugin BOM](https://github.com/jenkinsci/bom#readme) rather than depending on a specific version.
+
+* Before:
+    ```
+    <dependencies>
+      ...
+      <dependency>
+        <groupId>com.github.docker-java</groupId>
+        <artifactId>docker-java</artifactId>
+        <version>3.4.1</version>
+      </dependency>
+      ...
+    </dependencies>
+    ```
+* After:
+    ```
+    <dependencies>
+      ...
+      <dependency>
+        <groupId>org.jenkins-ci.plugins</groupId>
+        <artifactId>docker-java-api</artifactId>
+      </dependency>
+      ...
+    </dependencies>
+    ```