Add guidelines for answering into changeset discussions notifications

[Zverik]

A russian member of DWG writes: "A mapper who added a waterfall via MAPS.ME wrote to me, that he could not answer a comment in a changeset, because he did not visit the url, but tried to answer via e-mail, replying to a notification. And he was getting an error about an unknown sender. After that I tried adding an explanation to one of changeset comments: "To answer this, you should click a link to visit OpenStreetMap website. Then login using your credentials and type in your comment into an empty field in the Discussion section". I've got an answer literally in a couple of hours. I didn't test this again yet, but it worked right away."

So can we have either an explanation for replying to notifications, or a parser for replies via e-mail to these notifications? That means both changeset discussions and notes, I guess.

[tomhughes]

Patches welcome ;-)

[AntonKhorev]

With #4812 notifications say "Reply or learn more about the changeset at %{url}." and changeset pages say "Log in to join the discussion" + the same for notes. Maybe that's enough for this issue.