Update README.md

Author: iwasrobbed

README.md

@@ -1,5 +1,7 @@
 ## Objective-C Cheat Sheet
 
+Support this project via Gratipay <a href="https://gratipay.com/iwasrobbed/"><img src="http://img.shields.io/gratipay/iwasrobbed.svg"></a>
+
 **Note**: I wrote this over a stretch of 3 days, so it could probably use some editing and clarifications where necessary.
 
 This is not meant to be a beginner's guide or a detailed discussion about Objective-C; it is meant to be a quick reference to common, high level topics.  Please feel free to edit this document to update or improve upon it, making sure to keep with the general formatting of the document.  The list of contributors can be [found here](https://github.com/iwasrobbed/Objective-C-CheatSheet/graphs/contributors).