Add list of caveats for incomplete/absent features

[neiljp]

The client works rather well at this point, but it's important for prospective users to know whether features are present, hidden/undocumented/difficult-to-find, partially implemented, not implemented. These could refer to issues/PRs, and be put in various categories.

This probably belongs in the FAQ, though perhaps is significant enough to sit in a separate document. This may be too large to fit reasonably in the README, where I've seen it in other projects.

This was previously discussed in #zulip-terminal>launch checklist on chat.zulip.org.

[preetmishra]

I am inclined towards a separate document that we could link to FAQ.