Add a contribution guide

CONTRIBUTING.md

+# Contribution Guide
+
+Thank you for your interest in this mod! Before contributing, be sure to know
+about these few guidelines:
+
+- Contributions have to be under the zlib license (or compatible) for code,
+  and CC BY-SA 3.0 license (or compatible) for assets.
+- Make sure to update the change log, keeping the
+  [change log format](http://keepachangelog.com/) we use.
+- Don't bump the version yourself. Maintainers will do this when necessary.