Add a contribution guideline

CONTRIBUTING.md

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+# 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/kotlinx-benchmark/issues) for filing feature requests and bug reports.
+
+Questions about usage and general inquiries are better suited for StackOverflow or the #benchmarks channel in KotlinLang Slack.
+
+## Submitting changes
+
+Submit pull requests [here](https://github.com/Kotlin/kotlinx-benchmark/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 `master` branch.
+    * Base your PRs against the `master` branch.
+* 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.
+        * Depending on a particular bug, it may require either a unit or an integration test. For the latter, please check [integration module](integration/src/test) for examples.
+    * 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.
+* 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
+
+This project is built with Gradle.
+
+* Run `./gradlew build` to build. It also runs all the tests.
+
+You can import this project into IDEA, but you have to delegate build actions
+to Gradle (in Preferences -> Build, Execution, Deployment -> Build Tools -> Gradle -> Build and run).
+
+### Updating the public API dump
+
+* Use the [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 the updated API indexes together with other changes.