Updated user document for Single-Source RenderScript

Bug: 29875503
Bug: 29879448

Added a section introducing the new single-source feature.

Local staging:
http://yangni.mtv.corp.google.com/guide/topics/renderscript/compute.html

This updates
https://developer.android.com/guide/topics/renderscript/compute.html

Change-Id: I62dda3ab60b1678a9580fd2873f64f33d9696e13
(cherry picked from commit c069ed7f)