Add reference to Express 5 change

Author: aredridel

README.md

@@ -1,7 +1,12 @@
 engine-munger
 =============
 
-A replacement Express view class that provides asynchronous resolution, allows engines to use the lookup method to locate partials, and extends the lookup method to be configurable based on i18n locale and a template specialization rule map.
+A replacement Express view class that provides asynchronous resolution, allows
+engines to use the lookup method to locate partials, and extends the lookup
+method to be configurable based on i18n locale and a template specialization
+rule map.
+
+This is a backport of [work for Express 5](https://github.com/strongloop/express/pull/2653)
 
 Lead Maintainer: [Aria Stewart](https://github.com/aredridel)