Convert plaintext documents to Markdown

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
2 jobs for master in 54 seconds
Status Job ID Name Coverage
  Containers Build
passed #1212561
container_build

00:00:24

 
  Test
passed #1212562
test

00:00:29