Update common-mistakes.md (react-navigation#403)

* Update common-mistakes.md for wrapping AppContainer in View without flex

See github issue: react-navigation/react-navigation#5651

* Update common-mistakes.md

Author: napalm272

website/versioned_docs/version-3.x/common-mistakes.md

@@ -112,3 +112,37 @@ const AppContainer = createAppContainer(AppNavigator);
 
 In previous version of React Navigation, the library used to dig through your component tree to find `navigationOptions`. This is no longer the case, and only `navigationOptions` on screen components of a particular navigator will apply to that navigator. You can read more about this in the [Navigation options resolution](navigation-options-resolution.html) guide.
 
+## Wrapping AppContainer in a View without flex
+
+ If you wrap the `AppContainer` in a `View`, make sure the `View` is using flex.
+```javascript
+import React from 'react';
+import { Text, View } from 'react-native';
+import { createBottomTabNavigator, createAppContainer } from 'react-navigation';
+ class HomeScreen extends React.Component {
+  render() {
+    return (
+      <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
+        <Text>Home!</Text>
+      </View>
+    );
+  }
+}
+ class SettingsScreen extends React.Component {
+  render() {
+    return (
+      <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
+        <Text>Settings!</Text>
+      </View>
+    );
+  }
+}
+ const TabNavigator = createBottomTabNavigator({
+  Home: HomeScreen,
+  Settings: SettingsScreen,
+});
+ const AppContainer = createAppContainer(TabNavigator)
+ // without the style you will see a blank screen
+export default ()=><View style={{flex: 1}}><AppContainer/></View>;
+```
+