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Distribute glad/README.md, not nonexistent README

EXTRA_DIST in src/glad/Makefile.am included README, which doesn't exist.
Distribute README.md instead.  This fixes the `make dist` error:

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

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
3 jobs for dist-readme in 3 minutes and 39 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Container
passed #5858427
x86_build

00:01:32

 
  Build
passed #5858429
autotools

00:02:04

passed #5858428
cmake

00:01:24