README: Document support for LLVM/Clang Git main as of 2021-08-25
README: Document support for LLVM/Clang Git main as of 2021-08-25

CastXML already compiles and passes all tests against this version.

CastXML/CastXML,CastXML/CastXML,ellert/CastXML,bradking/CastXML,CastXML/CastXML,ellert/CastXML,bradking/CastXML,ellert/CastXML,bradking/CastXML