Add CONTRIBUTING file

CONTRIBUTING

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+This code tries to solve a particular problem with a very simple
+implementation. We try to keep the code to a minimum while making
+it as clear as possible. The design is very likely finished, and
+if some feature is missing it is possible that it was left out on
+purpose. That said, new usage patterns may arise, and when that
+happens we are ready to adapt if necessary.
+
+A good first step for contributing is to meet us on IRC and discuss
+ideas. We spend a lot of time on #lesscode at freenode, always ready
+to talk about code and simplicity. If connecting to IRC is not an
+option, you can create an issue explaining the proposed change and
+a use case. We pay a lot of attention to use cases, because our
+goal is to keep the code base simple. Usually the result of a
+conversation is the creation of a different tool.
+
+Please don't start the conversation with a pull request. The code
+should come at last, and even though it may help to convey an idea,
+more often than not it draws the attention to a particular
+implementation.