Add a CONTRIBUTING document

CONTRIBUTING

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+The preferred way to contribute would be through github pull requests,
+as an alternative patches can be discussed on the irc channel.
+
+When contributing your first changes, please include an empty commit for
+copyright waiver using the following message:
+
+  John Doe Copyright Waiver
+
+  I dedicate any and all copyright interest in this software to the
+  public domain.  I make this dedication for the benefit of the public at
+  large and to the detriment of my heirs and successors.  I intend this
+  dedication to be an overt act of relinquishment in perpetuity of all
+  present and future rights to this software under copyright law.