ux(docs): add an OpenCollective banner to readme

Closes #567.

README.md

@@ -11,9 +11,13 @@ and performance.
 [![npm downloads](https://img.shields.io/npm/dm/snabbdom.svg)](https://www.npmjs.com/package/snabbdom)
 [![Join the chat at https://gitter.im/paldepind/snabbdom](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/paldepind/snabbdom?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
+[![Donate to our collective](https://opencollective.com/snabbdom/donate/button@2x.png?color=blue)](https://opencollective.com/snabbdom#section-contribute)
+
 Thanks to [Browserstack](https://www.browserstack.com/) for providing access to
 their great cross-browser testing tools.
 
+* * *
+
 ## Introduction
 
 Virtual DOM is awesome. It allows us to express our application's view