Added link to the forum to CONTRIBUTING.md

.github/CONTRIBUTING.md

 
 # Help 
-#### The issues section is for bug reports and feature requests only. If you need help, please use [stackoverflow](http://stackoverflow.com/questions/tagged/three.js).
+#### The issues section is for bug reports and feature requests only. If you need help, please use the [forum](http://discourse.threejs.org/) or [stackoverflow](http://stackoverflow.com/questions/tagged/three.js).
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 # Bugs
 #### Before reporting a bug