[SPARK-6391][DOCS] Document Tachyon compatibility.

Adds a section in the RDD persistence section of the programming-guide docs detailing Spark-Tachyon version compatibility as discussed in [[SPARK-6391]](https://issues.apache.org/jira/browse/SPARK-6391).

Author: Calvin Jia <jia.calvin@gmail.com>

Closes #6382 from calvinjia/spark-6391 and squashes the following commits:

113e863 [Calvin Jia] Move compatibility info to the offheap storage level section.
7942dc5 [Calvin Jia] Add a section in the programming-guide docs for Tachyon compatibility.

docs/programming-guide.md

@@ -1214,9 +1214,11 @@ storage levels is:
     Compared to MEMORY_ONLY_SER, OFF_HEAP reduces garbage collection overhead and allows executors
     to be smaller and to share a pool of memory, making it attractive in environments with
     large heaps or multiple concurrent applications. Furthermore, as the RDDs reside in Tachyon,
-    the crash of an executor does not lead to losing the in-memory cache. In this mode, the memory 
+    the crash of an executor does not lead to losing the in-memory cache. In this mode, the memory
     in Tachyon is discardable. Thus, Tachyon does not attempt to reconstruct a block that it evicts
-    from memory.
+    from memory. If you plan to use Tachyon as the off heap store, Spark is compatible with Tachyon
+    out-of-the-box. Please refer to this <a href="http://tachyon-project.org/master/Running-Spark-on-Tachyon.html">page</a>
+    for the suggested version pairings.
   </td>
 </tr>
 </table>