added upgrade guid for RouteContext

Author: ryanflorence

upgrade-guides/v2.0.0.md

@@ -199,3 +199,26 @@ import { RoutingContext } from 'react-router'
 // v2.0.0
 import { RouterContext } from 'react-router'
 ```
+
+
+## `RouteContext` Mixin
+
+```js
+// 1.x
+const RouteComponent = React.createClass({
+  mixins: [ RouteContext ]
+})
+
+// 2.0.0
+const RouteComponent = React.createClass({
+  contextTypes: {
+    route: React.PropTypes.object
+  },
+  getChildContext() {
+    return {
+      route: this.props.route
+    }
+  }
+})
+```
+