This converts GOVERNANCE, MEMBERS and README to Markdown documents.
These are only cosmetic changes, the actual contents and wording have
been retained.

GitLab pretty-prints Markdown and adds anchors. We can now add links
from one document to another.

Unfortunately GOVERNANCE lettered lists have been converted to numbered
lists, because Markdown doesn't support the former.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: #3