squashed from https://gitlab.com/famedly/conduit/-/merge_requests/604
added differences.md
Co-authored-by: Charles Hall <charles@computer.surgery>
Co-authored-by: strawberry <strawberry@puppygock.gay>
Signed-off-by: strawberry <strawberry@puppygock.gay>
It is common to have a markdown file per deployment subdirectory.
Still install it as `README.Debian` to `/usr/share/doc/matrix-conduit`
as per Debian policy.
Also update the link in the main `README.md` file.