doc: add a file explaining the contribution process in detail

doc: add a file explaining the contribution process in detail

Add a separate document explaining the contribution process for libgpiod
with emphasis on the Developer's Certificate of Origin.

I based the text of this document on the one written by Grant Likely[1]
and adjusted it for libgpiod.

[1] https://github.com/glikely/obs-ptz/blob/main/CONTRIBUTING.md

Suggested-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Link: https://lore.kernel.org/r/20240416212141.6683-3-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>