Add a tip explaining how to enable-tor on NixOS

Add a tip explaining how to enable-tor on NixOS

This is a little bare bones but hopefully useful, some things that could
be improved:

1. Follow steps in order and replicate (I reconstructed off of my
   success, so there may be inaccuracies or things I forgot)

2. Clarify correct ownership for directories & sockets
   - enable-tor's sudo child will obtain the hostname so no access is
     needed after that
   - does it also create the tor sockety directory?

3. Remove unnecessary boilerplate
   - can the hidden service dir be left with the NixOS default?