Updated CONTRIBUTING.md.

CONTRIBUTING.md

@@ -1,7 +1,8 @@
 # Contributing
 
 Please do not make pull requests against master, any such pull requests will be
-closed. Instead make them against the dev branch.
+closed. Pull requests against the dev branch are accepted in some treebanks but
+not in others - check the Contributing line in the README file!
 
 For full details on the branch policy see
 [here](http://universaldependencies.org/release_checklist.html#repository-branches).