Add doc note for nesting wrapped navigator (react-navigation#1947)

After having to look for a solution for nesting a navigator embedded in a component, found this issue that dealt with the same problem: react-navigation#90.
Added the solution to the guide as a side note.

Author: nargov

docs/guides/Guide-Nested.md

@@ -72,3 +72,43 @@ Now we have put one navigator inside another, and we can `navigate` between navi
 ```phone-example
 nested
 ```
+
+## Nesting a Navigator in a Component
+Sometimes it is desirable to nest a navigator that is wrapped in a component. This is useful in cases where the navigator only takes up part of the screen. For the child navigator to be wired into the navigation tree, it needs the `navigation` property from the parent navigator.
+
+```js
+const SimpleApp = StackNavigator({
+  Home: { screen: NavigatorWrappingScreen },
+  Chat: { screen: ChatScreen },
+});
+```
+In this case, the NavigatorWrappingScreen is not a navigator, but it renders a navigator as part of its output.
+
+```js
+class NavigatorWrappingScreen extends React.Component {
+  render() {
+    return (
+      <View>
+        <SomeComponent/>
+        <MainScreenNavigator/>
+      </View>
+    );
+  }
+}
+```
+
+To wire `MainScreenNavigator` into the navigation tree, we assign its `router` to the wrapping component. This makes `NavigatorWrappingScreen` "navigation aware", which tells the parent navigator to pass the navigation object down. Since the `NavigatorWrappingScreen`'s `router` is overridden with the child navigator's `router`, the child navigator will receive the needed `navigation`.
+
+```js
+class NavigatorWrappingScreen extends React.Component {
+  render() {
+    return (
+      <View>
+        <SomeComponent/>
+        <MainScreenNavigator navigation={this.props.navigation}/>
+      </View>
+    );
+  }
+}
+NavigatorWrappingScreen.router = MainScreenNavigator.router;
+```