By having a ".md" extension, gitlab renders a nice page instead of
showing as plain text.

Currently our README is pretty bad. Partly, because it doesn't get
shown nicely.

Rename. The file effectively was already markdown. The old file is
gone.

For this we also need to change the automake flavor to "foreign"
(See [1]).

[1] https://autotools.info/automake/options.html#automake.options.flavors