Please, let this become a standard for every website, that '?' brings up a list of shortcut keys.
Edit: It works in Gmail!
I press '?' on any site that I think might/should have keyboard shortcuts!
However, I don't think that this should be solved at a website level. Websites shouldn't know or care what keys do in your browser; they should simply provide reasonable shortcut keys. Browsers should make it so that their shortcut keys cannot be overridden without your explicit permission.
Git diff supports the same.