readme: better android 12 explanation

Author: gotev

README.md

@@ -29,7 +29,10 @@ You are also safe if your app is put in the background. All the uploads will con
 Bear in mind that if you kill your app, the service gets killed as well, as it's attached to your app's process and all the currently running uploads will be terminated abruptly.
 
 ## Features <a name="features"></a>
-* Android 5.0 (API 21) to Android 12 (API 31) support. Beware you cannot start uploads while your app is the background on Android 12 (when your app is targeting API 31) due to recent Service limitations
+* Android 5.0 (API 21) to Android 12 (API 31) support. 
+  * *Android 12 Note*:
+    * What's supported: uploads initiated while the app is in foreground, with progress indication notification
+    * What's NOT supported: uploads started while the app is in the background or uploads without progress indication notification. This is due to the Service limitations imposed by Google, which requires all background services to display a notification to the user. Current architecture cannot support this. For support of those use-cases, WorkManager is the only option.
 * 100% Kotlin and fully interoperable with Java
 * upload files to a server with `FTP`, `HTTP multipart/form-data` or `Binary` data requests
 * upload requests can be serialized and executed later