docs: Add CONTRIBUTING.md.

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+Maintainer guidelines
+=====================
+
+Hello, and thank you for choosing to contribute to this repository. The short
+information below will help everyone to stop calling me a fascist:
+
+1. Always test your changes and write tests for them. The tests run
+   automatically and will tell you if there's a problem on any of the
+   supported platforms/versions.
+2. When committing, categorize what you did by following [Angular's Git Commit
+   Guidelines](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit).
+   Basically, prepend one of feat/fix/docs/style/refactor/perf/text/chore to the
+   commit subject, like so: `fix: Fix foo in bar.`. Capitalizing the first
+   letter and trailing periods are encouraged.
+3. Don't be an asshole.
+
+That's pretty much it, thanks for your contribution!