README: Update existing and future features

As the ecosystems around us evolve so should the NixOS mailserver project.

DKIM signing could be improved by allowing users to treat DKIM keys like a secret that they would commonly manage through agenix/sops/etc.

Forwarding mail these days requires SRS and possibly ARC. The latter has already become a required feature for bulk message to iCloud[1] and Google Mail[3]. I propose that we stay ahead of the curve by adding support for these features.

LDAP user management was added, but one pain point is that we currently prevent it from coexisting with declarative users.

And finally Oauth (via RFC7628[3]) is the new kid on the block that everyone wants to try out, but most notably client support[4] for hosting this yourself is not quite there yet.

[1] https://support.apple.com/en-us/102322 [2] https://support.google.com/a/answer/81126?hl=en#zippy=%2Crequirements-for-all-senders%2Crequirements-for-sending-or-more-messages-per-day [3] https://www.rfc-editor.org/rfc/rfc7628.html [4] https://bugzilla.mozilla.org/show_bug.cgi?id=1602166