ActivityPub Miscellaneous Terms draft

At our last SocialCG meeting, we proposed developing a context document 
and documentation for some extension properties and types often used in 
Activity Streams 2.0 documents, with the goal of testing out our new 
extensions policy <https://swicg.github.io/extensions-policy/>.

Based on input from that meeting, I've created the first draft of the 
documentation, ready for review:

https://swicg.github.io/miscellany/

Please read this over and give feedback in the form of GitHub issues, here:

https://github.com/swicg/miscellany/issues

Ideally I'd love to move this proposal quickly into a CG draft, so 
implementers could start using it quickly.

Evan