Add `NSPhotoLibraryAddUsageDescription` for Info.plist doc

I recently got issue in some version of iOS (apparently 10?) with this key missing.
Not sure why I needed that but as it feel redundant with the key below, but this avoid some crashes I found with sentry :)

README.md

 <string>Some description</string>
 <key>NSLocationWhenInUseUsageDescription</key>
 <string>Some description</string>
+<key>NSPhotoLibraryAddUsageDescription</key>
+<string>Some description</string>
 <key>NSPhotoLibraryUsageDescription</key>
 <string>Some description</string>
 <key>NSSpeechRecognitionUsageDescription</key>