GitLab

README.md: add "License and source code distribution" + logo

See merge request tails/tails!7

We did not decide to make this account official yet, so it feels weird to point
to it so prominently, even more so in a place that does not point to our
official Twitter account.

Otherwise, the "content" of that file (as tracked by Git)
is… the target of the symlink, which does not work too well.

When one uses the "/label ~xyz" syntax, in the web interface
one gets label completion after typing "~", which is not the case
when one uses the "/label xyz" syntax.

So let's hint contributors towards the former, more usable syntax, even if it
does not matter much here — where it's a matter of copying'n'pasting.

This page has no backlink on our website anymore.

A redirection was set up to handle links to that page, that may live elsewhere:
tails/puppet-tails@fdc2c10d

adding some links to readmine for better discoverability of content/contact methods that help people to migrate. 
related issue is: tails/tails#17722

The original link became a 404 after a GitLab upgrade, which was
a problem. 2a2089c5 fixed the 404,
although by making the link point to a different page than intended.

The previous link was broken for at least 2 different reasons.

While I'm at it:

 - switch this link to using the tails_gitlab ikiwiki shortcut
 - make the URL filter the labels it's about

Previously it linked to Starter issues in the tails/tails project,
but we have a few Starter issues in other tails/* projects.