Information on the 3 api clients. (#569)

Customer was confused around why there is 3 api clients. Can we remove the rest, src api clients from the repo? If not let's put an explanation as to how these 3 api clients differ, and recommend to always use the cloud-client if possible.

Author: cobookman

bigquery/README.md

@@ -7,6 +7,14 @@ Client Libraries.
 For more information, read the [Getting Started with BigQuery and the Google Java API Client
 library][1] codelab.
 
+## API Libraries
+- cloud-client (Preferred Option)
+  - This is Google Cloud's Official API Client, and the recommended way to interact with BQ
+- rest
+  - This shows java code implementing a sample client by making use of BQ's RESTful API. 
+- src
+  - This client was generated by running the veneer on the BQ protobuf definition. It demonstrates how you can build client libraries against our apiary service even if an official client library does not exist.
+
 ## Quickstart
 
 Install [Maven](http://maven.apache.org/).