sys/README.md: source roadmap improvements

Tweak the existing descriptions slightly. Add entries for several
directories which are not yet documented, but not exhaustively.

Reviewed by: imp (previous version), emaste
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37133

(cherry picked from commit 397013385d4b88ee7e6d9b13bd8c017ca9dfdad5)