Update mapview.md (#2171)

Add 'none' option to docs for mapType

docs/mapview.md

@@ -9,7 +9,7 @@
 | `initialRegion` | `Region` |  | The initial region to be displayed by the map.  Use this prop instead of `region` only if you don't want to control the viewport of the map besides the initial region.<br/><br/> Changing this prop after the component has mounted will not result in a region change.<br/><br/> This is similar to the `initialValue` prop of a text input.
 | `mapPadding` | `EdgePadding` |  | Adds custom padding to each side of the map. Useful when map elements/markers are obscured. **Note** Google Maps only.
 | `liteMode` | `Boolean` | `false` | Enable lite mode. **Note**: Android only.
-| `mapType` | `String` | `"standard"` | The map type to be displayed. <br/><br/> - standard: standard road map (default)<br/> - satellite: satellite view<br/> - hybrid: satellite view with roads and points of interest overlayed<br/> - terrain: (Android only) topographic view<br/> - mutedStandard: more subtle, makes markers/lines pop more (iOS 11.0+ only)
+| `mapType` | `String` | `"standard"` | The map type to be displayed. <br/><br/> - standard: standard road map (default)<br/> - none: no map<br/> - satellite: satellite view<br/> - hybrid: satellite view with roads and points of interest overlayed<br/> - terrain: (Android only) topographic view<br/> - mutedStandard: more subtle, makes markers/lines pop more (iOS 11.0+ only)
 | `customMapStyle` | `Array` |  | Adds custom styling to the map component. See [README](https://github.com/airbnb/react-native-maps#customizing-the-map-style) for more information.
 | `showsUserLocation` | `Boolean` | `false` | If `true` the app will ask for the user's location. **NOTE**: You need to add `NSLocationWhenInUseUsageDescription` key in Info.plist to enable geolocation, otherwise it is going to *fail silently*!
 | `userLocationAnnotationTitle` | `String` | | The title of the annotation for current user location. This only works if `showsUserLocation` is true. There is a default value `My Location` set by MapView. **Note**: iOS only.