Merge pull request #1956 from bfirsh/link-to-zenhub

Link to ZenHub instead of Waffle

CONTRIBUTING.md

@@ -58,6 +58,6 @@ you can specify a test directory, file, module, class or method:
 
 ## Finding things to work on
 
-We use a [Waffle.io board](https://waffle.io/docker/compose) to keep track of specific things we are working on and planning to work on. If you're looking for things to work on, stuff in the backlog is a great place to start.
+We use a [ZenHub board](https://www.zenhub.io/) to keep track of specific things we are working on and planning to work on. If you're looking for things to work on, stuff in the backlog is a great place to start.
 
 For more information about our project planning, take a look at our [GitHub wiki](https://github.com/docker/compose/wiki).