README.md: shift description of kernel config files

Document it in sys/README.md instead. Describe the purpose of LINT
config files as well.

Reviewed by: imp
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37134

(cherry picked from commit 9e0dcd0ab296ddcdee60a4952435175ae3f7bdb3)