CONTRIBUTING.md: add initial contributing guideline

CONTRIBUTING.md: add initial contributing guideline

Document some of the practises and expectations that we've been using
in the project.

Large chunks of the document are inspired by wlroots - thank you o/

Going further, I think we'd want to document the use of SoB/DCO and
mention about our CI pipelines, so that people try them before opening
PRs/sending patches.

Although I think that can follow-up at another day.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/122
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>