Add a CLA.

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+## Contributing Code
+
+You are welcome to contribute code to G13GUI in order to fix bugs or implement new features.
+
+There are three important things to know:
+
+  1. You must be aware of the (LICENSE)[license] this project is licensed
+     under, and *agree to the Contributors License Agreement*. This is common
+     practice in all major Open Source projects.
+
+  2. *Not all proposed contributions will be accepted*.
+
+## Contributor License Agreement
+
+When you contribute (code, documentation, or anything else) you have to be
+aware that your contribution is covered by the same licnse that is applied
+to G13GUI itself.
+
+This applies to all contributors, including those contributing on behalf of
+a company. If you agree to its content, you simply have to write "I agree
+to the Contributor License Agreement" in a comment on your pull request.
+