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
I liked the "at a minimum" from before to say that a descriptor requires a only a schema. It would also be better to say what a schema does. It is the definition of the dataset's records.