readme: point to github wiki (#627)
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
vixns/marathon-lb,mesosphere/marathon-lb,vixns/marathon-lb,mesosphere/marathon-lb,vixns/marathon-lb,mesosphere/marathon-lb