Update contribution policy (#175)

Author: fzhinkin

CONTRIBUTING.md

@@ -14,6 +14,12 @@ Submit pull requests [here](https://github.com/Kotlin/kotlinx-io/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.