note about plugins

Author: hakimel

CONTRIBUTING.md

@@ -17,3 +17,7 @@ When reporting a bug make sure to include information about which browser and op
   - Single-quoted strings
 - Should be made towards the **dev branch**
 - Should be submitted from a feature/topic branch (not your master)
+
+
+### Plugins
+Please do not submit plugins as pull requests. They should be maintained in their own separate repository. More information here: https://github.com/hakimel/reveal.js/wiki/Plugin-Guidelines