Add Uninstall docs

[BenWhitehead]

@ssk2 Please review. /cc @air

[air]

I think this is a little cryptic: Zookeeper Node provided to CASSANDRA_ZK Using zkCli run the rmr command

1. how do I find that value CASSANDRA_ZK
2. where can I find zkCli, on what machine do I run it, what rmr args do I need.

This is great btw, will serve as reference example for others.

[BenWhitehead]

@ssk2 I incorporated all your proposed edits and added some of the notes you mentioned. Please review.

@air In the case of DCOS I've added in the default values for the rmr command. The point about how to find the values I think plays into "How do we expose configuration values for Services", WDYT?

[ssk2]

One small correction, otherwise good!