Moving Kite Dataset API info from old guide per CDK-753.

Link to the old guide from which this came. This is just shards of information brought forward so they're not lost. This info should be woven into the samples before 1.0.

http://kitesdk.org/docs/current/kite-data/guide.html
Immutability is more of a statement about sharing the object. An immutable object doesn't store any state between calls, they are independent. In this case, it means you can instantiate one Dataset and use it simultaneously in different threads. But that's probably a little to down in the weeds for this doc.