Update README

<3 @elcuervo
WIP: add Ruster::Cluster
Add Ruster::Cluster, a wrapper to Redis Cluster administration
Cluster#add_node

This is probably the most awful set of tests I can come up with. I need
to take another look at these tests, as I'm quite sure that during this
streak all tests will be this awful.
Add Node#ip and Node#port convenience methods
WIP: remove node from cluster