Add basic instructions on how to contribute to OfflineIMAP

I discovered in the pull request #95 that the development branch is
`next`, not `master`.

To prevent this kind of errors, GitHub shows a useful bright yellow banner
if a CONTRIBUTING.md file is present in the repository.
See <https://github.com/blog/1184-contributing-guidelines>.

I based this skeleton file on the instructions found at
<http://offlineimap.org/development.html>.

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+How to contribute to OfflineIMAP
+================================
+
+There are several branches.
+
+`master` represents the current stable branch or what will be the next
+sable release.
+
+`next` is where development happens. This is what will be merged into
+`master` for the next major release.
+
+Finally, `pu` is a feature branch which hosts crazy features that might or
+might not make it into some release. This branch could easily be reset at any
+point in time.
+
+To contribute a patch
+---------------------
+
+1. Create a feature branch based on `next`:
+
+        $ git checkout next
+        $ git branch my-feature
+        $ git checkout my-feature
+
+2. Commit your awesome changes:
+
+        $ git commit -m "Add my important feature to the interface"
+
+3. Make a pull request on GitHub or send a patch to the mailing list.
+
+Additional information
+----------------------
+
+Other instructions on how to contribute to OfflineIMAP are available at
+<http://offlineimap.org/development.html>.