Add the contribution guideline (Kotlin#180)

* Add the contribution guideline

Author: fzhinkin

CONTRIBUTING.md

@@ -0,0 +1,47 @@
+# Contributing Guidelines
+
+One can contribute to the project by reporting issues or submitting changes via pull request.
+
+## Reporting issues
+
+Please use [GitHub issues](https://github.com/Kotlin/binary-compatibility-validator/issues) for filing feature requests and bug reports.
+
+Questions about usage and general inquiries are better suited for StackOverflow or the [#library-development](https://kotlinlang.slack.com/archives/C8C4JTXR7) channel in KotlinLang Slack.
+
+## Submitting changes
+
+Submit pull requests [here](https://github.com/Kotlin/binary-compatibility-validator/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.
+
+* All development (both new features and bug fixes) is performed in the `develop` branch.
+    * The `master` branch contains the sources of the most recently released version.
+    * Base your PRs against the `develop` branch.
+    * The `develop` 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 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 that 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.
+* 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 has received positive feedback. Propose a solution if none has been suggested.
+
+## Building
+
+In order to build and run tests in the project in IDE, two prerequisites are required:
+
+* Java 11 or above in order to use the latest ASM
+* All build actions in the IDE should be delegated to Gradle
+
+To run tests with Gradle, you can execute `./gradlew check`. That will run both unit and functional tests, and it also will check the public API changes.
+
+### Updating the public API dump
+
+* Run following commands to update the public API:
+    * Run `./gradlew apiDump` to update API index files.
+    * Commit the updated API indexes together with other changes.

README.md

@@ -248,3 +248,7 @@ In order to build and run tests in the project in IDE, two prerequisites are req
 
 * Java 11 or above in order to use the latest ASM
 * All build actions in the IDE should be delegated to Gradle
+
+## Contributing
+
+Read the [Contributing Guidelines](CONTRIBUTING.md).