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
Historically, libnm claimed to be LGPL-2.0+ licensed. But as keep mixing that
with LGPL-2.1+ code (e.g. from systemd), so probably even the parts that
claimed to be LGPL-2.0+, were not entirely correctly doing so.
Anyway, since we switched to SPDX license identifiers, we now claim
everywhere that the right license is LGPL-2.1+. Update the comment to
reflect that.
Also drop the paragraph about "autoconf mechanism". The general
guideline is self evident, while it didn't mention meson builds.
Also, a first time contributor likely won't likely be concerned about
this, as this is already advanced.
