sys/README.md: Add a section for documentation

Add web links to intro(9) and the Architecture Handbook.

Reviewed by: lwhsu, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41253

(cherry picked from commit 32e47f62724e6cce2df0bc69d6ceba2d058d3585)