readme: updated contributing section

README.md

@@ -21,7 +21,7 @@ If you find this image useful here's how you can help:
 
 - Send a Pull Request with your awesome new features and bug fixes
 - Help new users with [Issues](https://github.com/sameersbn/docker-browser-box/issues) they may encounter
-- Send me a tip via [Bitcoin](https://www.coinbase.com/sameersbn) or using [Gratipay](https://gratipay.com/sameersbn/)
+- Support the development of this image with a [donation](http://www.damagehead.com/donate/)
 
 # Installation