Mention not accepting new addons/modes in CONTRIBUTING.md

CONTRIBUTING.md

@@ -47,6 +47,10 @@ should be asked on the
 
 ## Contributing code
 
+Note that we are not accepting any new addons or modes into the main
+distribution. If you've written such a module, please distribute it as
+a separate NPM package.
+
 - Make sure you have a [GitHub Account](https://github.com/signup/free)
 - Fork [CodeMirror](https://github.com/codemirror/CodeMirror/)
   ([how to fork a repo](https://help.github.com/articles/fork-a-repo))