docs(firebase_dynamic_links): update readme with up-to-date info (#2692)

Co-authored-by: Mike Diarmid <mike.diarmid@gmail.com>

Author: Sam3077

packages/firebase_dynamic_links/README.md

@@ -81,9 +81,9 @@ You can receive a Dynamic Link containing a deep link that takes the user to spe
 
 1. In the [Firebase Console](https://console.firebase.google.com), open the Dynamic Links section.
   - Accept the terms of service if you are prompted to do so.
-  - Take note of your project's Dynamic Links URL prefix, which is displayed at the top of the Dynamic Links page. You need your project's Dynamic Links URL prefix to programmatically create Dynamic Links. A Dynamic Links URL prefix looks like `https://YOUR_SUBDOMAIN.page.link`.
+  - Take note of your project's Dynamic Links URL prefix, which is displayed at the top of the Dynamic Links page. You need your project's Dynamic Links URL prefix to programmatically create Dynamic Links. Unless you are using a custom domain, a Dynamic Links URL prefix looks like `https://YOUR_SUBDOMAIN.page.link`.
 
-Receiving dynamic links on *iOS* requires a couple more steps than *Android*. If you only want to receive dynamic links on *Android*, skip to step 4. You can also follow a video on the next two steps [here.](https://youtu.be/sFPo296OQqk?t=2m40s)
+Receiving dynamic links on *iOS* requires a couple more steps than *Android*. If you only want to receive dynamic links on *Android*, skip to step 5. You can also follow a video on the next two steps [here.](https://youtu.be/KLBjAg6HvG0?t=60)
 
 2. In the **Info** tab of your *iOS* app's Xcode project:
   - Create a new **URL Type** to be used for Dynamic Links.
@@ -92,10 +92,23 @@ Receiving dynamic links on *iOS* requires a couple more steps than *Android*. If
 3. In the **Capabilities** tab of your app's Xcode project, enable **Associated Domains** and add the following to the **Associated Domains** list:
 
 ```
-applinks:YOUR_SUBDOMAIN.page.link
+applinks:YOUR_URL_PREFIX
 ```
 
-4. To receive a dynamic link, configure listeners for link callbacks when the application is active or in background calling `onLink` and call the `getInitialLink()` method from `FirebaseDynamicLinks` which gets the link that opened the app (or null if it was not opened via a dynamic link).
+Remember not to include `https://` or any slashes or paths in your prefix
+
+4. **If you are using a custom domain** create an key in your `Info.plist` file called `FirebaseDynamicLinksCustomDomains` and set it to your app's Dynamic Link URL prefixes. For example:
+
+```xml
+<key>FirebaseDynamicLinksCustomDomains</key>
+<array>
+  <string>https://example.com/promos</string>
+  <string>https://example.com/links/share</string>
+</array>
+```
+
+5. To receive a dynamic link, call the `getInitialLink()` method from `FirebaseDynamicLinks` which gets the link that opened the app (or null if it was not opened via a dynamic link)
+and configure listeners for link callbacks when the application is active or in background calling `onLink`.
 
 ```dart
 void main() {