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
We might also mention that a `Dataset` is [immutable](https://jsr-305.googlecode.com/svn/trunk/javadoc/javax/annotation/concurrent/Immutable.html).