Further editing down of Porting Guide.
Further editing down of Porting Guide.

Combined two sections that repeated information in the Porting Guide.

takahito-tejima/OpenSubdiv,shengfuintel/OpenSubdiv,takahito-tejima/OpenSubdiv,shengfuintel/OpenSubdiv,lookup1980/OpenSubdiv,lookup1980/OpenSubdiv,lookup1980/OpenSubdiv,nyotis/OpenSubdiv,c64kernal/OpenSubdiv,c64kernal/OpenSubdiv,nyotis/OpenSubdiv,anshuarya/OpenSubdiv,anshuarya/OpenSubdiv,xldeveloper/OpenSubdiv,c64kernal/OpenSubdiv,xldeveloper/OpenSubdiv,nyotis/OpenSubdiv,xldeveloper/OpenSubdiv,takahito-tejima/OpenSubdiv,shengfuintel/OpenSubdiv,anshuarya/OpenSubdiv