Update CONTRIBUTING.md

Author: lmccart

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 ### The github issues are for bugs and feature requests for the p5.js library itself. If you have a general question or bug programming with p5.js please post it in the [p5.js forum](https://forum.processing.org/two/).
 
+### Please make sure you are posting to the right repository. See this [section](https://github.com/processing/p5.js/blob/master/README.md#issues) for a list of all p5.js repositories.
+
 ## p5.js Code of Conduct
 
 * **Be mindful of your language.** Any of the following behavior is unacceptable: