README: mentioned nginx.org changelog

Fixes #491

README.md

@@ -7,3 +7,4 @@ regarding contributing and issues.
 The full readme is generated over in [docker-library/docs](https://github.com/docker-library/docs),
 specifically in [docker-library/docs/nginx](https://github.com/docker-library/docs/tree/master/nginx).
 
+The changelog for NGINX releases is available at [nginx.org changes page](https://nginx.org/en/CHANGES).