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 think this can be removed. The first sentence is confusing because the dataset is not itself an Avro object and it refers to "the datastore". The second sentence might be valuable later on.