readme: changes should be submitted via GitLab

510188250ea8 ("Add governance document") adds a GOVERNANCE document
describing development based on GitLab merge requests. Update the README
file accordingly.

Some information is duplicated across README and GOVERNANCE, this is
intentional to make README provide a more human-friendly, less
bureaucratic version. GOVERNANCE is still the authoritative version.

Signed-off-by: Simon Ser <contact@emersion.fr>