Update CONTRIBUTING.md

Summary: Includes a new line that says to open an issue before making a non-libdef or non-doc contribution.

Reviewed By: nmote

Differential Revision: D13864371

fbshipit-source-id: e5491f76e2dc64e03889abf6dc21a434ae2abd8c

CONTRIBUTING.md

@@ -1,5 +1,10 @@
-We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you have any questions, please drop us a line at cla@fb.com. 
+We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you have any questions, please drop us a line at cla@fb.com.
 
-You are also expected to follow the [Code of Conduct](CODE_OF_CONDUCT.md), so please read that if you are a new contributor. 
+You are also expected to follow the [Code of Conduct](CODE_OF_CONDUCT.md), so please read that if you are a new contributor.
+
+---
+
+If you intend to contribute something that is NOT a library definition or documentation change,
+please open a "discussion" issue first to ensure that your work aligns with the Flow team's goals.
 
 Thanks!