Add README.md to EXTRA_DIST

Otherwise it isn't included in the generated tarballs.

Suggested-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
11 jobs for README.md-EXTRA_DIST in 1 minute and 34 seconds (queued for 2 seconds)