Update README.md for clarity

[VR-Nima]

Added context for certain lines to make them more clear, i.e. "head-tracking" added to 6dof for HMD movement to differentiate from 6dof motion controls, "frontend GUI" changed to "UEVRInjector.exe" for those who don't know what a frontend or GUI are, and made the subject more clear in the instructions for HDR so people are aware that the line is referring to HDR being on.