Only UTC timezone for ISO 8601

[nearwall]

Hello! I use humantime-serde to work with json, it works fine!

But I was a bit confused with it behavior with ISO 8601 formatted strings without timezone. Strings like that: "2022-07-12T12:01:48" are interpreted like UTC time zone. After that I checked the documentation for rfc3339. There are the same additional notices for all rfc3339 functions: "The value is always UTC and ignores system timezone".

Maybe this crucial information could be noticed in 'Features:' for rfc3339 ? It could save time for many ~~careless(like me)~~ developers

I greatly would appreciate your response and time taken to write it. =)