Add note about what to do when you get a webmention

[sandeepshetty]

• Don't show content as a comment by default. List the link as a mention instead
• Only for u-in-reply-to does it make sense to show the content as a comment.

[barnabywalters]

Sounds like this was closed already, if anyone feels a5e6785 didn’t adequately resolve this feel free to reopen