CONTRIBUTING: remove autotools instructions, add more information about meson
Summary
This commit updates the CONTRIBUTING.md file to remove information about Autotools and add some additional tips for using meson.
Purpose
As we are deprecating autotools, I think it's a good idea to start guiding people towards using meson and also offering some guidance for users that have only used autotools so far.
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
-
the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable -
the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.