Add Shawn McCarney as a maintainer

Matt Spinler suggested that I add myself as a maintainer since I'll be
contributing a lot of code for the new phosphor-regulators application
in this repository.

Change-Id: I00a56ff28c7632113a368a66e331b0199d17c746
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 9dee4c9..a7f4598 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -43,3 +43,4 @@
 -------------------------
 
 M:  Matt Spinler <spinler@us.ibm.com> <mspinler[m]!>
+M:  Shawn McCarney <shawnmm@us.ibm.com> <ShawnMcCarney!>