docs: Make it clear that the app must be registered to show live ads …

…in the published app. (#325)

Co-authored-by: jjliu15 <[email protected]>

packages/google_mobile_ads/README.md

@@ -23,7 +23,7 @@ inline banner and native ads.
     *   Android Gradle Plugin 4.1 or higher (this is the version supported by Flutter out of the box)
 *   Ios
     *   Latest version of Xcode with [enabled command-line tools](https://flutter.dev/docs/get-started/install/macos#install-xcode).
-*   Recommended: [Create an AdMob account](https://support.google.com/admob/answer/2784575) and [register an Android and/or iOS app](https://support.google.com/admob/answer/2773509)
+*   Recommended: [Create an AdMob account](https://support.google.com/admob/answer/2784575) and [register an Android and/or iOS app](https://support.google.com/admob/answer/2773509) (To show live ads on a published app, it is required to register that app).
 
 
 ## Import the Mobile Ads SDK
@@ -1711,4 +1711,4 @@ load to fail. In particular, for errors under the domain `com.google.admob` on
 iOS and `com.google.android.gms.ads` on Android, the `GetMessage()` can be
 looked up in [this help center
 article](//support.google.com/admob/answer/9905175) for a more detailed
-explanation and possible actions that can be taken to resolve the issue.
+explanation and possible actions that can be taken to resolve the issue.