DOC: add Ben Shillito as the maintainer of 51d

you can put me down as the maintainer with this email.

This should be backported to 1.6.
diff --git a/MAINTAINERS b/MAINTAINERS
index 06fca15..ea0060a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -58,3 +58,6 @@
 Maintainer: David Carlier <dcarlier@afilias.info>
 Files: src/da.c, include/*/da.h
 
+51Degrees device identification
+Maintainer: Ben Shillito <ben@51degrees.com>
+Files: src/51d.c