add note about transitions

closes #1931

Author: ryanflorence

UPGRADE_GUIDE.md

@@ -113,21 +113,6 @@ since its simple with a `*` path.
 <Link to={`/users/${user.id}`}>Mateusz</Link>
 ```
 
-### RouteHandler
-
-`RouteHandler` is gone. `Router` now automatically populates
-`this.props.children` of your components based on the active route.
-
-```js
-// v0.13.x
-<RouteHandler/>
-<RouteHandler someExtraProp={something}/>
-
-// v1.0
-{this.props.children}
-{React.cloneElement(this.props.children, {someExtraProp: something })}
-```
-
 #### "active" class
 
 In 0.13.x links added the "active" class by default which you could
@@ -173,6 +158,21 @@ active when the index route is active.
 <IndexLink to="/">Home</IndexLink>
 ```
 
+### RouteHandler
+
+`RouteHandler` is gone. `Router` now automatically populates
+`this.props.children` of your components based on the active route.
+
+```js
+// v0.13.x
+<RouteHandler/>
+<RouteHandler someExtraProp={something}/>
+
+// v1.0
+{this.props.children}
+{React.cloneElement(this.props.children, {someExtraProp: something })}
+```
+
 ### Navigation Mixin
 
 If you were using the `Navigation` mixin, use the `History` mixin instead.
@@ -269,6 +269,32 @@ we've realized that we can build a better implementation on top of the
 router and will be doing that very soon, before the 1.0 final release,
 but it doesn't need to be baked into routing like it was before.
 
+### `willTransitionTo` and `willTransitionFrom`
+
+Routes now define this behavior:
+
+```js
+// v0.13.x
+var Home = React.createClass({
+  statics: {
+    willTransitionTo (transition, params, query, callback) {
+    }
+    willTransitionFrom (component, transition, params, query, callback) {
+    }
+  }
+})
+
+// v1.0
+<Route
+  component={Home}
+  onEnter={(location, replaceWith) => {}}
+  onLeave={() => {}}
+/>
+```
+
+To cancel a "transition from", please refer to the
+[Confirming Navigation](docs/advanced/ConfirmingNavigation.md) guide.
+
 ### We'll keep updating this
 
 There's a lot of the old API we've missed, please give the [new