MAINTAINERS: add Serge Hallyn as a credentials reviewer

Also add the documentation file as suggested by Günther Noack.

Signed-off-by: Serge Hallyn <sergeh@kernel.org>
[PM: subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: sergeh@kernel.org

MAINTAINERS

@@ -6141,11 +6141,13 @@ F:	drivers/hid/hid-creative-sb0540.c
 
 CREDENTIALS
 M:	Paul Moore <paul@paul-moore.com>
+R:	Serge Hallyn <sergeh@kernel.org>
 L:	linux-security-module@vger.kernel.org
 S:	Supported
 T:	git https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/lsm.git
 F:	include/linux/cred.h
 F:	kernel/cred.c
+F:	Documentation/security/credentials.rst
 
 INTEL CRPS COMMON REDUNDANT PSU DRIVER
 M:	Ninad Palsule <ninad@linux.ibm.com>