Improved documentation about zh-* deprecation and upgrade path

> <+bmispelon>	 bouke: in my mind, the purpose of the release notes is twofold: 1) allow you to tell if you're affected by a change in django 2) help you go through an upgrade
>  <+bmispelon>	 the current release note is good for 1), but I think we can do better for 2)