Distribute glad/README.md, instead of nonexistent README

EXTRA_DIST in src/glad/Makefile.am includes README, which doesn't exist. Distribute README.md instead to fix the make dist error:

make[6]: *** No rule to make target 'README', needed by 'distdir-am'.  Stop.
...
make: *** [Makefile:648: dist] Error 2

Thanks for considering,
Kevin