Update README.md

Add PayPal donation button

README.md

-<img src="https://raw.githubusercontent.com/junegunn/i/master/fzf.png" height="170" alt="fzf - a command-line fuzzy finder"> [![travis-ci](https://travis-ci.org/junegunn/fzf.svg?branch=master)](https://travis-ci.org/junegunn/fzf)
+<img src="https://raw.githubusercontent.com/junegunn/i/master/fzf.png" height="170" alt="fzf - a command-line fuzzy finder"> [![travis-ci](https://travis-ci.org/junegunn/fzf.svg?branch=master)](https://travis-ci.org/junegunn/fzf) [![Donate via PayPal](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=EKYAW9PGKPD2N)
 ===
 
 fzf is a general-purpose command-line fuzzy finder.