Add NSCameraUsageDescription example

For Android people it's not obvious why app crashes.

README.md

@@ -101,6 +101,8 @@ In order to use this plugin, add the following to your Info.plist file:
 ```
 <key>io.flutter.embedded_views_preview</key>
 <true/>
+<key>NSCameraUsageDescription</key>
+<string>This app needs camera access to scan QR codes</string>
 ```
 
 ## Flip Camera (Back/Front)
@@ -137,4 +139,4 @@ Requires at least SDK 24 (Android 7.0).
 # Credits
 * Android: https://github.com/zxing/zxing
 * iOS: https://github.com/mikebuss/MTBBarcodeScanner
-* Special Thanks To: LeonDevLifeLog for his contributions towards improving this package.
+* Special Thanks To: LeonDevLifeLog for his contributions towards improving this package.