Added CONTRIBUTING file as discussed in mrdoob#2774.

CONTRIBUTING.md

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+# 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).
+
+## How to report a bug?
+
+* The revision number of three.js
+* Tell us where did you observe the problem: your browser version, your operating system, your graphics card (for example Chrome 23.0.1271.95, Windows 7, Nvidia Quadro 2000M)
+* If you can, try to check your problem on multiple browsers / multiple computers (if you are on Windows, especially helpful is to [compare ANGLE and OpenGL renderers](https://github.com/mrdoob/three.js/wiki/How-to-use-OpenGL-or-ANGLE-rendering-on-Windows))
+* Identify the problem. Saying "it doesn't work" doesn't help much.
+* A small testcase (big chance is that when you do this, you'll find the answer yourself).
+* What did you try already?
+
+## How to contribute to three.js?
+
+* Make sure you have a GitHub account.
+* Fork the repository on GitHub.
+* Check [contribution guidelines](https://github.com/mrdoob/three.js/wiki/How-to-contribute-to-three.js).
+* Make changes to your clone of the repository.
+* Submit a pull request.
+