Adds version-sync comment to Soy rule

Change-Id: If411fbbca6ac99f3c62474cbe68ddc0f7aa450fd

lib/soy/BUCK

diff --git a/lib/soy/BUCK b/lib/soy/BUCK
index c92c810..a55edd0 100644
--- a/lib/soy/BUCK
+++ b/lib/soy/BUCK
@@ -1,3 +1,4 @@
+# Keep this version of Soy synchronized with the version used in Gerrit.
 maven_jar(
   name = 'soy',
   id = 'com.google.template:soy:2016-08-09',