Add guidelines as to when commands should be in WebDriver v. other specs

[gsnedders]

Based on a discussion within the WPT core team, it would be good to have some guidance as to when things belong in the WebDriver spec versus when they should be defined in other specs. (This discussion was originally about #1695, but this is a more general question.)

@jgraham's suggestion was anything that deeply ties into other specs should be defined in those other specs, but of course in theory this also applies to things like the WebDriver navigation commands.

It would be good to have some documentation for contributors as to whether they should add a WebDriver endpoint to WebDriver v. put it in another spec.