add pretty-format-snabbdom to testing section

Author: jeysal

README.md

@@ -165,6 +165,7 @@ http://oleg.fi/graafi/
 ### Testing
 
 * [erykpiast/cyclejs-mock ★22](https://github.com/erykpiast/cyclejs-mock) - Utility for testing applications based on CycleJS framework.
+* [jeysal/pretty-format-snabbdom ★0](https://github.com/jeysal/pretty-format-snabbdom) - A [pretty-format](https://github.com/facebook/jest/tree/master/packages/pretty-format) ([Jest](https://facebook.github.io/jest/) snapshot) plugin for rendering snapshots of snabbdom VNodes as nicely as those of React elements
 
 ### Debugging