Remove Wayland build depenedency and update README.Debian 

The libwayland-dev build depends was not sufficient to actually enable Wayland support. You also need libxkbcommon-dev, which is not available in older distributions, so just remove libwayland-dev as a dependency, and add a section to README.Debian describing how to enable Wayland support by manually installing sufficiently recent packages.

Also add a section to README.Debian describing the differences with the official Debian package.

I think I've fixed the merging problem. At least it's giving me a green "able to merge" icon as I type this.