Add sponsor link to CONTRIBUTING.md

[ci skip]

Author: josegonzalez

CONTRIBUTING.md

@@ -11,6 +11,7 @@ several ways to help out:
 * Contribute to the [documentation](http://dokku.viewdocs.io/dokku/)
 * Come up with new ways to show off our [lovely logo](https://avatars1.githubusercontent.com/u/13455795)
 * Blog about different ways you are using dokku
+* [Sponsor](https://opencollective.com/dokku#support) the Dokku project financially 
 
 There are a few guidelines that we need contributors to follow so that we have
 a chance of keeping on top of things.