Document what source code mirroring does about userstyles.world

[vednoc]

I'll add more info about it in the FAQ.

[vednoc]

Will any of the metadata that was included when I imported the style be updated?

I've been thinking of using "Mirror style metadata" option for that, since ATM it's only used for mirroring metadata from USo-archive. @Gusted I'd like know what you think about this.

Is my Preview URL also checked and updated?

No. Not entirely sure if we should do this, and if can it be done efficiently. I think @Gusted would be better at answering this question.

[vednoc]

Style metadata (except license field) will now be mirrored. I left out the license field until I figure out how to deal with garbage input. My initial idea was to add a select menu and let users select it from there, but we'll see.