Add blog post for 4.x release

Author: satya164

website/blog/2019-09-16-react-navigation-4.0.md

@@ -0,0 +1,23 @@
+---
+title: React Navigation 4.0
+author: Core Team
+authorURL: https://twitter.com/reactnavigation
+---
+
+The documentation is now live at https://reactnavigation.org, and v3 lives [here](/docs/en/3.x/getting-started.html).
+
+In this release, we have removed the navigators from the `react-navigation` package. The navigators have lived in separate packages for a quite a while and you could already use those packages manually, but we still bundled them in the `react-navigation` package. This resulted in the bundled navigators not being up-to-date.
+
+With this version, you now install the navigators from their respective packages, which means that you can independently update them.
+
+The navigators live at:
+
+- `createStackNavigator` - [`react-navigation-stack`](https://github.com/react-navigation/stack)
+- `createBottomTabNavigator`, `createMaterialTopTabNavigator` - [`react-navigation-tabs`](https://github.com/react-navigation/tabs)
+- `createDrawerNavigator` - [`react-navigation-drawer`](https://github.com/react-navigation/drawer)
+
+For upgrade instructions, please check the [release notes](https://github.com/react-navigation/react-navigation/releases/tag/v4.0.0).
+
+---
+
+Thanks for reading, please post any issues you encounter to https://github.com/react-navigation/react-navigation/issues!