Updating Readme
Updating Readme

Adding usage option for --use-vulkan etc. Also documented use of multi-backend api
FlorentGuinier/RadeonRays_SDK,GPUOpen-LibrariesAndSDKs/RadeonRays_SDK,FlorentGuinier/RadeonRays_SDK,beasterio/FireRays_SDK,beasterio/FireRays_SDK,josh04/RadeonRays_SDK,GPUOpen-LibrariesAndSDKs/RadeonRays_SDK,GPUOpen-LibrariesAndSDKs/RadeonRays_SDK,beasterio/FireRays_SDK,generalistr6/Radeon_Rays,generalistr6/Radeon_Rays,FlorentGuinier/RadeonRays_SDK,josh04/RadeonRays_SDK,generalistr6/Radeon_Rays,josh04/RadeonRays_SDK