Document how to provide contracts from flatpak apps

## 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.