add mf/mf/discussions link to issue homepage and make `registration_announce` issue more actionable

[bumblefudge]

As discussed in #109 . Don't merge until discussions are turned on for Multiformats/multiformats, tho!

[rvagg]

discussions on, would you like to have the honours of first post? https://github.com/multiformats/multiformats/discussions/new?category=announcements&welcome_text=true