README: add Gittip link.

Closes Homebrew#20482.

README.md

@@ -23,10 +23,17 @@ License
 -------
 Code is under the [BSD 2 Clause (NetBSD) license][license].
 
+Donations
+---------
+We accept tips through [Gittip][tip].
+
+[![Gittip](http://img.shields.io/gittip/Homebrew.png)](https://www.gittip.com/Homebrew/)
+
 [home]:http://brew.sh
 [wiki]:http://wiki.github.com/Homebrew/homebrew
 [mxcl]:http://twitter.com/mxcl
 [formula]:http://github.com/Homebrew/homebrew/tree/master/Library/Formula/
 [braumeister]:http://braumeister.org
 [license]:https://github.com/Homebrew/homebrew/tree/master/Library/Homebrew/LICENSE
 [team]:https://github.com/Homebrew?tab=members
+[tip]:https://www.gittip.com/Homebrew/