Description of contributing procedure made more appropriate for the way we develop Truffle

Author: Jaroslav Tulach

CONTRIBUTING.md

@@ -1,2 +1,12 @@
-Please see http://openjdk.java.net/contribute/ for details of how to contribute
-to OpenJDK projects including GraalVM.
+To submit pull requests for truffle, you need to sign the [Oracle Contributor Agreement][1].
+There is a white-list for contributors who have signed the OCA so please add a comment
+to your first pull request indicating the name you used to sign the OCA if it isn't clear
+from your github profile.
+
+Pull requests can only be merged by committers and a committer cannot merge
+his or her own pull requests.
+
+When creating a pull request, make sure you designate an assignee to ensure
+the request is processed in a timely manner.
+
+[1]: http://www.oracle.com/technetwork/community/oca-486395.html