Document dualscreeninfo deviceOrientation property

[kmelmon]

This PR introduced a new prop for react-native-dualscreeninfo:
#34

The prop is DualScreenInfo.orientation

It has values from the below enum:

export enum DeviceOrientation {
Portrait = 'portrait',
Landscape = 'landscape',
PortraitFlipped = 'portraitFlipped',
LandscapeFlipped = 'landscapeFlipped',
};

We should update the spec with this prop.