Update readme
Update readme

* The TressFX project no longer depends on AGS.
* Reference the thumbnail from github.io instead of directly in the
repo.

mrgreywater/TressFX,GPUOpen-Effects/TressFX,GPUOpen-Effects/TressFX,vlj/TressFX,GPUOpen-Effects/TressFX,mrgreywater/TressFX,mrgreywater/TressFX,vlj/TressFX,vlj/TressFX