Extend installtion docs in readme

Some Linux Mint user got the following error message:

error: Can't load uri https://flatpak.dorfbrunnen.eu/repo/appstream/app.briar.gtk.flatpakref: Peer sent fatal TLS alert: Error in protocol version

After enabling TLS 1.2 on flatpak.dorfbrunnen.eu, they got:

error: The application app.briar.gtk/x86_64/master requires the runtime org.gnome.Platform/x86_64/3.36 which was not found

They fixed it by manually adding the repos.

Related to #50 (closed).