feat: add intialRouteName to deep-linking guide

Author: WoLewicki

website/versioned_docs/version-5.x/deep-linking.md

@@ -39,10 +39,7 @@ For example, the path `/rooms/chat?user=jane` will be translated to a state obje
 The [`useLinking`](use-linking.html) hook makes it easier to handle incoming links:
 
 ```js
-import {
-  NavigationContainer,
-  useLinking,
-} from '@react-navigation/native';
+import { NavigationContainer, useLinking } from '@react-navigation/native';
 
 function App() {
   const ref = React.useRef();
@@ -168,7 +165,7 @@ Here we have a stack navigator in root, and inside the `Home` screen of the root
 }
 ```
 
-In this config, we specify that the `Profile` screen should be resolved for the `users/:id` pattern and it's nested inside the `Home` screen. This will result in the following state object:
+In this config, we specify that the `Profile` screen should be resolved for the `users/:id` pattern and it's nested inside the `Home` screen. Then parsing `users/jane` will result in the following state object:
 
 ```js
 const state = {
@@ -188,6 +185,46 @@ const state = {
 };
 ```
 
+Sometimes we want to ensure that a certain screen will always be present for the navigator in the state object. We can achieve it by specifying the `initialRouteName` property for that navigator in the config. For the above example, if we want the `Notifications` screen to be the initial route in `Home` tab navigator, we should specify such config (`Notifications` screen needn't be mentioned in `screens` property):
+
+```js
+{
+  Home: {
+    initialRouteName: 'Notifications',
+    screens: {
+      Profile: 'users/:id',
+      Notifications: 'notify/:user',
+    },
+  },
+};
+```
+
+Then, the path `/users/42` will resolve to the following state object:
+
+```js
+const state = {
+  routes: [
+    {
+      name: 'Home',
+      state: {
+        index: 1,
+        routes: [
+          {
+            name: 'Notifications',
+          },
+          {
+            name: 'Profile',
+            params: { id: 'jane' },
+          },
+        ],
+      },
+    },
+  ],
+};
+```
+
+Notice that we can't pass any params to the `Notifications` screen if it isn't explicitly mentioned in the URL string, so the screen should implement handling lack of these params with e.g. providing default ones.
+
 For some advanced cases, specifying the mapping may not be sufficient. You can implement your custom parser to address these cases using the `getStateFromPath` option:
 
 ```js