* How to use MIX_ENV

* How to create a user

* Link to certbot's site with the webform which advises how to obtain
  a certificate based on web server/OS.

Also remove the erlang packages, they're specified in the Debian guide.

$ mix phx.serve
** (Mix) The task "phx.serve" could not be found. Did you mean "phx.server"?

$ mix phx.server
[info] Running Pleroma.Web.Endpoint with Cowboy using http://0.0.0.0:4000

This reverts commit 7b2bded0.