Update UPGRADE_GUIDE.md

Author: ryanflorence

UPGRADE_GUIDE.md

@@ -14,8 +14,6 @@ anymore. Our mixins like `State` and `Navigation` just proxied calls to
 some methods on an undocumented feature of React called `context`, that
 in turn called methods on the router instance under the hood.
 
-Note, you can still use our mixins, you'll just get a deprecation warning.
-
 Without mixins we needed a way for you to get access to these methods.
 We decided the simplest solution was to stop hiding the router instance
 and just put the whole thing on context.
@@ -25,6 +23,8 @@ tree that parent components (`Handler` in the `Router.run` callback) can
 explicitly define and descendent components can explicitly ask for. The
 stuff on context doesn't show up in a component unless you ask for it.
 
+**Note:** You can still use our mixins, you'll just get a deprecation warning.
+
 ```js
 // 0.12.x
 var Foo = React.createClass({