Adds section about background modes on iOS to the README (#1709)

Author: TimHoogstrate

geolocator/CHANGELOG.md

@@ -1,3 +1,7 @@
+## NEXT
+
+- Adds section about `UIBackgroundModes` to the README. 
+
 ## 14.0.1
 
 - Adds section about `FOREGROUND_SERVICE_LOCATION` to the README. 

geolocator/README.md

@@ -123,6 +123,15 @@ The second key (in this example called `YourPurposeKey`) should match the purpos
 > NOTE: the first time requesting temporary full accuracy access it might take several seconds for the pop-up to show. This is due to the fact that iOS is determining the exact user location which may take several seconds. Unfortunately this is out of our hands.
 </details>
 
+On iOS 16 and above you need to specify `UIBackgroundModes` `location` to receive location updates in the background.
+
+``` xml
+<key>UIBackgroundModes</key>
+<array>
+  <string>location</string>
+</array>
+```
+
 <details>
 <summary>macOS</summary>