MAINTAINERS: Add the security document to SECURITY CONTACT

When changing the document related to kernel security workflow, notify
the security mailing list as its concerned by this.

Cc: <security@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200827182029.3458-1-krzk@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>