A discussion at xmpp:conversations@conference.siacs.eu yielded, that
previous to this there was no documentation or in-app information about
the meaning of the check marks.

Two of the recommended plugins got merged in 1.4, and don’t exist any longer,
and when using flatpak the plugins must be installed that way.

* Create migrating_to_new_device.md

* Add link to Readme for further information.

* Added some info

* Make separate backup guide and integrate Readme information with new guides.

Co-authored-by: Licaon_Kter <licaon-kter@users.noreply.github.com>

fixes #4053

* various spelling fixes

* Fix typo

* Update README.md