docs: Add a basic security document

Based loosely on the Linux kernel
Documentation/admin-guide/security-bugs.rst file, create a basic
security document for U-Boot.  In sum, security issues should be
disclosed in public on the mailing list if at all possible as an initial
position.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>