Yes seconding what @sam said. Good ? help key support is an absolute must. Here's Twitter:
Beyond that, I am not sure this needs to be a plugin @sam I think it should be a core feature we always support and ship. I cannot see any reason whatsoever I would want to be on another Discourse forum anywhere on the Internet and see broken, non-functional, or non-standard keyboard shortcuts as I have come to expect on every Discourse forum.
(Unless explicitly disabled by the forum operator, of course, which is fine.. but on by default, shipped by default.)