The https://wiki.gnome.org/Projects/gvfs/debugging page contains info
about getting logs, but it is hard to find it. Let's add the link into
the README file to make easier.

The security-related issues should be reported over
https://security.gnome.org/ currently. Let's replace the obsolete
security@gnome.org e-mail address.

README file still points to bugzilla.gnome.org instead of gitlab.gnome.org.
Let's fix that and add some info about reporting security issues also.

Rename README to README.md to be shown nicely in GitLab intreface.
Add general info about reporting bugs and ask questions.