Repository has no description in readmine, markdown and html aren't enabled

Markdown and html are currently disabled for the readmine file. Making it look as contributors might expect, won't be possible until it is enabled.

While migration is still going on, it might be still useful to add a couple of URLs for better discoverability of content/contact methods that helps people to migrate.

A readmine file with some usefull links to begin with is here: https://gitlab.tails.boum.org/syster/tails/-/blob/master/README