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)
Status Job ID Name Coverage
  Build
passed #44045
xserver-1.13

00:00:29

passed #44046
xserver-1.14

00:00:29

passed #44047
xserver-1.15

00:00:30

passed #44048
xserver-1.16

00:00:30

passed #44049
xserver-1.17

00:00:28

passed #44050
xserver-1.18

00:00:26

passed #44051
xserver-1.18-clang

00:00:30

passed #44052
xserver-1.19

00:00:26

passed #44053
xserver-1.19-clang

00:00:29

passed #44043
xserver-1.20

00:00:39

passed #44044
xserver-1.20-clang

00:00:32