Update README.md (#19)

Add `onErrorImg` prop to documentation

README.md

@@ -95,6 +95,7 @@ class App extends React.Component {
 | `onCropChange` | Function | ✓ | Called everytime the crop is changed. Use it to update your `crop` state.|
 | `onZoomChange` | Function |  | Called everytime the zoom is changed. Use it to update your `zoom` state. |
 | [`onCropComplete`](#onCropCompleteProp) | Function |  | Called when the user stops moving the image or stops zooming. It will be passed the corresponding cropped area on the image in percentages and pixels |
+| `onImgError` | Function |  | Called when error occurs while loading an external image |
 | `style` | `{ containerStyle: object, imageStyle: object, cropAreaStyle: object }` |  | Custom styles to be used with the Cropper. Styles passed via the style prop are merged with the defaults. |
 | `classes` | `{ containerClassName: string, imageClassName: string, cropAreaClassName: string }` |  | Custom class names to be used with the Cropper. Classes passed via the classes prop are merged with the defaults. |