Add contributing document.

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Author: mrward

CONTRIBUTING.md

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+# Contributing to SharpDevelop
+
+Thanks for helping to improve SharpDevelop.
+
+In order for your changes to be accepted you can either sign the [Joint Copyright Assignment](http://www.icsharpcode.net/TechNotes/JointCopyrightAssignment.pdf) or add the following statement to your pull request:
+
+_I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the #develop open source product (the "Contribution"). My Contribution is licensed under the MIT License._
+
+Unless we have a Joint Copyright Agreement on file or this statement is in your pull request, we cannot accept it.
+
+More information is available on [joining the team](https://github.com/icsharpcode/SharpDevelop/wiki/Joining-the-Team).