Added a Flattr button to the README file.

This has been requested by at least 10 people now, so it might be time
for one.

README.mdown

@@ -129,3 +129,17 @@ those questions to accept the (sane) default suggestions.
   
   For support branches, the `<base>` arg must be a commit on `master`.
 
+
+Showing your appreciation
+=========================
+A few people already requested it, so now it's here: a Flattr button.
+
+Of course, the best way to show your appreciation for the original
+[blog post](http://nvie.com/git-model) or the git-flow tool itself remains
+contributing to the community.  If you'd like to show your appreciation in
+another way, however, consider Flattr'ing me:
+
+[![Flattr this][2]][1]
+
+[1]: http://flattr.com/thing/53771/git-flow
+[2]: http://api.flattr.com/button/button-static-50x60.png