Mention that GPS metadata needs ACCESS_MEDIA_LOCATION permission on Android

[albertogasparin]

It took me ages to figure out that the library was loosing GPS metadata because of specific Android permissions (basically the values would turn up to be non rational numbers / Unknown).

I know this library is not Android specific, but it might be very valuable to people integrating it in their Android app to know that this is a requirement to read GPS metadata, and that is not a bug of the library.

[StefanOltmann]

You’re absolutely right! I’m sorry that I didn’t write that out.