Update README.md

Added T3D 3.5 and Project Manager 2.1 links.
Fix: Missing file Engine/source/sfx/openal/linux/LoadOAL.linux.cpp
Fix Linux compilation.
Change GFXDevice::getStateBlock() to return mNewStateBlock.
Bullet physics applyImpulse() fix

Bullet physics requires a local origin be passed into it's
applyImpulse() rather than a world origin.
Merge pull request #571 from DavidWyand-GG/BulletImpulseFix

Bullet physics applyImpulse() fix
Revert trigger polyhedron change

Reverts the change made in
https://github.com/GarageGames/Torque3D/pull/511.  See
http://www.garagegames.com/community/forums/viewthread/136033 for the
discussion.
Merge pull request #573 from DavidWyand-GG/TriggerFix

Revert trigger polyhedron change
Update version reporting to 3.5.1
Merge pull request #574 from DavidWyand-GG/Updates

Update version reporting to 3.5.1
Update README.md
Merge branch 'master' into development
Update README.md

Added T3D 3.5.1 link.
Add SDL2 library.
Basic SDL2 window & input support.
Add SDL2 message box.
SDL Fix: SDLWindowManager::_process don't check if window is valid.
SDL: Improve window support.
SDL: Improve mouse support.
SDL: Improve keyboard support.