Create documentation for HLSL/GLSL onramp path

[swoods-nv]

I ran into trouble trying to set up a toy project that uses Slang to ingest existing GLSL shaders. It would be good to provide a section of documentation for on-ramp compatibility which covers the way to set up compiler options to read a GLSL (and presumably HLSL) shader as-is. (At least for the compilation API; it may be sufficiently documented for the command line.)