Updated CONTRIBUTING.md to describe new naming scheme.

CONTRIBUTING.md

@@ -74,4 +74,9 @@ broken into separate service objects.  These services map directly to how
 the [GitHub API documentation][] is organized, so use that as your guide for
 where to put new methods.
 
+Sub-service (e.g. [Repo Hooks][]) implementations are split into separate files
+based on the APIs they provide. These files are named service_api.go (e.g. 
+repos_hooks.go) to describe the API to service mappings. 
+
 [GitHub API documentation]: http://developer.github.com/v3/
+[Repo Hooks]: http://developer.github.com/v3/repos/hooks/