Merge request: add instructions to allow file sharing in Signal Desktop on Tails

Hey, I wrote an howto on Signal and Tails years ago, merged it with Bisco's, and more recently found yours and adopted it for the people I train.

Recently sharing files became blocked by perms (or i missed it before).

The solution is simple and implemented in this fork of your wiki: http://wmj5kiic7b6kjplpbvwadnht2nh2qnkbnqtcv3dyvpqtz7ssbssftxid.onion/gagz/messengers-on-tails-os/-/wikis/Signal/Install-Signal-desktop/diff?version_id=4da2dd4b3837707c27dc983868a0b5052c632b16

Please merge it or correct it before if you like :)

Also, i didn't find a simpler way to submit a merge request.