Move readme.txt to README.md

Signed-off-by: Alban VIDAL <alban.vidal@zordhak.fr>
2 jobs for social_contract_readme_markdown in 1 minute and 41 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Deploy
passed #132843
pages

00:01:41

passed pages:deploy

00:00:18