readme: point to github wiki (#627)

Links `Using marathon-lb` to a page on the github wiki instead of
`docs.mesosphere.com` so the links can be updated without a PR when the
URLs change.

Fixes: #559

README.md

@@ -22,7 +22,7 @@ fast, efficient, battle-tested, highly available load balancer with many advance
 
 ### Getting Started
 
- * [Using marathon-lb](https://docs.mesosphere.com/services/marathon-lb/mlb-basic-tutorial/)
+ * [Using marathon-lb](https://github.com/mesosphere/marathon-lb/wiki/Using-Marathon-LB)
  * [Securing your service with TLS/SSL (blog post)](https://mesosphere.com/blog/2016/04/06/lets-encrypt-dcos/)
 
 Take a look at [the marathon-lb wiki](https://github.com/mesosphere/marathon-lb/wiki) for example usage, templates, and more.