Add an example on how to use the onEnter callback

This change adds an example on how to use the onEnter callback to redirect a user to another page.

Author: thaiphan

docs/API.md

@@ -345,6 +345,28 @@ Called when a route is about to be entered. It provides the next router state an
 
 If `callback` is listed as a 3rd argument, this hook will run asynchronously, and the transition will block until `callback` is called.
 
+###### `callback` signature
+`cb(err)`
+
+```js
+const userIsInAnOrganisation = (nextState, replace, callback) => {
+  fetch(...)
+    .then(response = response.json())
+    .then(userOrganisations => {
+      if (userOrganisations.length === 0) {
+        replace({
+          pathname: '/users/' + nextState.params.userId + '/organisations/new',
+          state: { nextPathname: nextState.location.pathname }
+        })
+        
+        callback(null);
+      }
+    })
+}
+
+<Route path="/users/:userId/organisations" onEnter={userIsInAnOrganisation} />
+```
+
 ##### `onChange(prevState, nextState, replace, callback?)`
 Called on routes when the location changes, but the route itself neither enters or leaves. For example, this will be called when a route's children change, or when the location query changes. It provides the previous router state, the next router state, and a function to redirect to another path. `this` will be the route instance that triggered the hook.