Add support for @2x in TitleContainer foriOS
This allows Retina versions of files loaded via ContentManager and for files accessed via TitleContainer.
Update README.md

Changed the link to the contributing guide to the new CONTRIBUTING.MD.  Fixed remaining link with 'develop3d' in it.
Merge pull request #1440 from slygamer/titlecontainer-2x

Add support for @2x in TitleContainer for iOS