Correctly document how to build OpenSSH with mlock

[perturbing]

Note that to preserve mlocking, we need to tunnel the socket with a version of OpenSSH that is build with this build flag (see here)

 * sshd(8): add support for locking sshd into memory, enabled with
   the --with-linux-memlock-onfault c

On nix, this is easy, as per this attribute.

Given the various ways we build (see #43 ), we should create documentation on how to integrate this requirement in the setup.