Add donate button

README.md

@@ -2,6 +2,7 @@
 [![Build Status](https://travis-ci.org/jollheef/out-of-tree.svg?branch=master)](https://travis-ci.org/jollheef/out-of-tree)
 [![Go Report Card](https://goreportcard.com/badge/code.dumpstack.io/tools/out-of-tree)](https://goreportcard.com/report/code.dumpstack.io/tools/out-of-tree)
 [![Documentation Status](https://readthedocs.org/projects/out-of-tree/badge/?version=latest)](https://out-of-tree.readthedocs.io/en/latest/?badge=latest)
+[![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=R8W2UQPZ5X5JE&source=url)
 
 # [out-of-tree](https://out-of-tree.io)