added DefaultRoute documentation

Author: ryanflorence

docs/api/README.md

@@ -4,6 +4,7 @@ React Router API
 - [`Router`](/docs/api/Router.md)
 
 - Components
+  - [`DefaultRoute`](/docs/api/components/DefaultRoute.md)
   - [`Link`](/docs/api/components/Link.md)
   - [`Redirect`](/docs/api/components/Redirect.md)
   - [`Route`](/docs/api/components/Route.md)

docs/api/components/DefaultRoute.md

@@ -0,0 +1,62 @@
+API: `DefaultRoute` (component)
+===============================
+
+A route that is active when the parent route's path matches exactly. Or,
+in other words, the default child route for a parent.
+
+Note, this is not a `NotFoundRoute`. It is only active when the parent's
+route path is matched exactly.
+
+Props
+-----
+
+### `handler`
+
+The component to be rendered when the route is active.
+
+### `preserveScrollPosition`
+
+If `true`, the router will not scroll the window up when the route is
+transitioned to. Defaults to `false`. Ignored if the parent `<Routes/>`
+has been set to `true`.
+
+Example
+-------
+
+```xml
+<Routes>
+  <Route path="/" handler={App}>
+
+    <!--
+      when the url is `/`, this handler will be active, or in other
+      words, will be `this.props.activeRouteHandler in the `App` handler
+    -->
+    <DefaultRoute handler={Home}/>
+
+    <Route name="about" handler={About}/>
+    <Route name="users" handler={Users}>
+      <Route name="user" handler={User} path="/user/:id"/>
+
+      <!-- when the url is `/users`, this will be active -->
+      <DefaultRoute handler={UsersIndex}/>
+
+    </Route>
+  </Route>
+</Routes>
+```
+
+This is all really just a shortcut for the less intuitive version of the
+same functionality:
+
+```xml
+<!-- no path or name on what was previously the "users" route -->
+<Route handler={Users}>
+  <!-- the path moved down to the child -->
+  <Route name="users-index" path="/users" handler={UsersIndex}/>
+  <Route name="user" handler={User} path="/user/:id"/>
+</Route>
+```
+
+`DefaultRoute` feels more natural, so you can name and transition to the
+parent route.
+