Clarify README to talk about "local" versus "remote" host or computer

[hpk42]

feedback from fediverse https://social.jsteuernagel.de/@jana/114755565875220910
"Only thing of note is that I didn't understand until half way through setup that the script deploys chatmail to a remote server. So I ended up having the server deploy itself. :neobot_woozy:"