We should write our CONTRIBUTING files in markdown syntax, because
it's nice to read a plain text and gets nicely rendered.
However, if the file doesn't have a ".md" extension, gitlab's
web interface shows it as plain text file.
Rename the file.
This possibly breaks links like [1], but referring to a branch name
(and not a commit ID or a tag) is anyway fragile. Hence, I don't try
to fix that by adding a symlink or similar, because I think that just
makes it more confusing.
[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/master/CONTRIBUTING