Document how image region names, types and roles are extracted

[lourot]

Document which ones get extracted and how they are put together

See https://github.com/Frameright/image/blob/master/src/Metadata/Xmp/ImageRegion.php#L98

[lourot]

Also see now https://github.com/Frameright/php-image-metadata-parser/blob/master/src/Image.php#L86