Add some notes to CONTRIBUTING.md.

CONTRIBUTING.md

@@ -1,5 +1,8 @@
-We <3 Pull Requests! Only two things:
+We <3 Pull Requests! Three core things:
 
  1. If you are adding functionality or fixing a bug, please add a test!
  2. Add your name to AUTHORS.txt
  3. Use the PEP8 style guide.
+
+ If you want to add methods to the parser tree, we will need to discuss this in
+ an issue first.