am c161f20e: Merge change 4135 into donut

Merge commit 'c161f20e49717e8b1879173197e3528a39e12ccc'

* commit 'c161f20e49717e8b1879173197e3528a39e12ccc':
  Updated Platform Developers Guide welcome page, changing focus from 'porting guide' to 'platform developer guide'.  Modelled on the SDK site developers guide.