Add contribution guidelines

README.markdown

@@ -74,6 +74,24 @@ directory.  Edit a file from the repository.
 Fugitive cares about the current file, not the current working
 directory.  Edit a file from the repository.
 
+Contributing
+------------
+
+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 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 VimScript 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.
+
 Self-Promotion
 --------------
 
@@ -82,4 +100,4 @@ Like fugitive.vim? Follow the repository on
 [vim.org](http://www.vim.org/scripts/script.php?script_id=2975).  And if
 you're feeling especially charitable, follow [tpope](http://tpo.pe/) on
 [Twitter](http://twitter.com/tpope) and
-[Github](http://github.com/tpope).
+[GitHub](http://github.com/tpope).