cordova-plugin-geolocation no more working on iOS 11

The apache plugin cordova-plugin-geolocation is fully reliable on android but now bugged on iOS caused by missed properties injections in the plist file:
NSLocationAlwaysAndWhenInUseUsageDescription and NSLocationWhenInUseUsageDescription have to be precised on the offcial Apple documentation.

Those entries can now be added directly on the plugin installation by requesting a IOS_USAGE_DESCRIPTION variable.

To fix this problem, a few lines has to be added on the plugin.xml, and the readme has to be updated.