Add documentation for RenderScript changes

Summary:
- Document the new 'kernel' attribute
- Mention RenderScript support in the Release Notes.

Reviewers: rsmith

Subscribers: tberghammer, danalbert, cfe-commits, srhines

Differential Revision: http://reviews.llvm.org/D21212

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273283 91177308-0d34-0410-b5e6-96231b3b80d8