Create sys/README.md

Add this primarily to document the sys/ subdirectories of the source tree.

This is a straight copy from the contents of hier(7). Improvements will
follow in other changes.

Reviewed by: imp, emaste
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37132

(cherry picked from commit d1087fe8436dbe369b218854fffe901ccc7602c1)