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

MAINTAINERS

@@ -15546,6 +15546,7 @@ F:	include/uapi/linux/sed*
 SECURITY CONTACT
 M:	Security Officers <security@kernel.org>
 S:	Supported
+F:	Documentation/admin-guide/security-bugs.rst
 
 SECURITY SUBSYSTEM
 M:	James Morris <jmorris@namei.org>