docs: Add contributing file

Author: RichardLitt

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing
+
+Source hosted at [GitHub](http://github.com/splitrb/split-analytics).
+Report Issues/Feature requests on [GitHub Issues](http://github.com/splitrb/split-analytics/issues).
+
+Please follow the [Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Note on Patches/Pull Requests
+
+ * Fork the project.
+ * Make your feature addition or bug fix.
+ * Add tests for it. This is important so I don't break it in a
+   future version unintentionally.
+ * Commit, do not mess with rakefile, version, or history.
+   (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
+ * Send me a pull request. Bonus points for topic branches.
+
+## Testing
+
+Tests can be ran with `rake spec`.`