Document how to provide contracts from flatpak apps

[peteruithoven]

Problem

With the move to Flatpak current approaches of providing contracts no longer work.

Proposal

Document how to provide contracts from apps distributed through Flatpak.

Relevant: https://github.com/elementary/hig/pull/33

@Marukesu mentioned:

Well, they can, they only need to add --talk-name=org.elementary.Contractor to finish args. We can also add a portal to copy the sandboxed contract to the host path so they can be used on others apps.