Compare Desktop's features with Android's one

Until Briar Desktop has the same feature set as Briar Android, we could show a comparison matrix on the Desktop download page that explains what features are supported in Briar Desktop.

changed milestone to %Desktop 0.2.0

added Document label

added Sponsor 7 label

assigned to @nico

mentioned in merge request !83 (merged)

added S: in code review label

unassigned @nico

mentioned in commit 6c3c1e59

closed with merge request !83 (merged)

added R: fixed label and removed S: in code review label