Add CONTRIBUTING document.
Fixed a problem with the IsDisposed Implementation in Gamer.cs
Merge remote-tracking branch 'upstream/develop' into gamerfix
Merge pull request #1439 from dellis1972/gamerfix

Fixed issue with Gamer IsDisposed implememtation
Add support for @2x in TitleContainer foriOS
This allows Retina versions of files loaded via ContentManager and for files accessed via TitleContainer.
Add check for GL_EXT_texture_filter_anisotropic before texture anisotropy used
Code review action - fix incorrect logic and adding missing GL error check
Merge pull request #1442 from Aranda/no_tex_aniso

Add check for GL_EXT_texture_filter_anisotropic before using anisotropic filtering
Merge pull request #1438 from slygamer/CONTRIBUTING

Add CONTRIBUTING document.
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