Add a note in the documentation about disabling offsetof warning.
Add a note in the documentation about disabling offsetof warning.

Because client code inlines Hbr, this warning is spurriously triggered by
several compilers and needs to be disabled within the client code build system.

fixes #227

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