doc: self-protection: provide initial details

This document attempts to codify the intent around kernel self-protection
along with discussion of both existing and desired technologies, with
attention given to the rationale behind them, and the expectations of
their usage.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
[jc: applied fixes suggested by Randy]
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>