Add a CONTRIBUTING file

Leave the README section in place, as we link to it from all over the
place.

CONTRIBUTING.markdown

@@ -0,0 +1,14 @@
+If your [commit message sucks](http://stopwritingramblingcommitmessages.com/),
+I'm not going to accept your pull request.  I've explained very politely
+dozens of times that
+[my general guidelines](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
+are absolute rules on my own repositories, so I may lack the energy to
+explain it to you yet another time.  And please, if I ask you to change
+something, `git commit --amend`.
+
+Beyond that, don't be shy about asking before patching.  What takes you
+hours might take me minutes simply because I have both domain knowledge
+and a perverse knowledge of Vim script so vast that many would consider
+it a symptom of mental illness.  On the flip side, some ideas I'll
+reject no matter how good the implementation is.  "Send a patch" is an
+edge case answer in my book.