[Nut-upsuser] I-D: ISE request for more detail on command STARTTLS

[Roger Price]

The IETF Independent Submissions Editor (ISE) has asked for more detail on the 
command STARTTLS, in particular the use of certificates.

I propose saying that NUT 2.8.0 supports the encryption of communications 
between Attachment Daemon upsd and Management Daemon upsmon using TLS 1.3 
[RFC8446] with X.509 v3 certificates as defined by RFC5280 + updates.

I also propose adding a note that in the closely restrained world of UPS 
management, it may be possible to obtain better security using self-signed 
certificates.

Aa always with RFC work, comments are welcome.  Roger