Add and install README.Debian

This file documents how the packaging is organized and how to
configure and use it.  It also details what the default deployment
is like.

Cargo.toml

@@ -86,6 +86,7 @@ section = "net"
 priority = "optional"
 assets = [
   ["debian/env.local", "etc/matrix-conduit/local", "644"],
+  ["debian/README.Debian", "usr/share/doc/matrix-conduit/", "644"],
   ["README.md", "usr/share/doc/matrix-conduit/", "644"],
   ["target/release/conduit", "usr/sbin/matrix-conduit", "755"],
 ]