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.

Looks like this should've happened as part of !479.

And offer help since it's pretty easy but impossible if you don't have
Nix installed.

Ruma requires Rust 1.64

So we need to re-enable some things. See their changelog[0] for details.

[0]: https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#migrating

We were inserting one too many 0xff bytes

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