Add comment about X-Forwarded-Proto mapping

nginx.tmpl

@@ -1,3 +1,5 @@
+# If we receive X-Forwarded-Proto, pass it through; otherwise, pass along the
+# scheme used to connect to this server
 map $http_x_forwarded_proto $proxy_x_forwarded_proto {
   default $http_x_forwarded_proto;
   ''      $scheme;