Adding rein to contributors.

PiperOrigin-RevId: 188929713
diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 9158fed..d33de98 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -21,3 +21,4 @@
 Nicholas Levin <nglevin@google.com>
 Thomas Van Lenten <thomasvl@google.com>
 Steven E Wright <stevewright@google.com>
+Rein Hillmann <rein@google.com>