docs(camera-preview): add usage

closes #363

src/plugins/camera-preview.ts

@@ -21,6 +21,52 @@ export interface CameraPreviewSize {
  *
  * For more info, please see the [Cordova Camera Preview Plugin Docs](https://github.com/cordova-plugin-camera-preview/cordova-plugin-camera-preview).
  *
+ * @usage
+ * ```
+ * import { CameraPreview } from 'ionic-native';
+ *
+ * // camera options (Size and location)
+ * let cameraRect: CameraPreviewRect = {
+ *   x: 100,
+ *   y: 100,
+ *   width: 200,
+ *   height: 200
+ * };
+ *
+ *
+ * // start camera
+ * CameraPreview.startCamera(
+ *   cameraRect, // position and size of preview
+ *   'front', // default camera
+ *   true, // tape to take picture
+ *   false, // disable drag
+ *   true // send the preview to the back of the screen so we can add overlaying elements
+ * );
+ *
+ * // Set the handler to run every time we take a picture
+ * CameraPreview.setOnPictureTakenHandler().subscribe((result) => {
+ *   console.log(result);
+ *   // do something with the result
+ * });
+ *
+ *
+ * // take a picture
+ * CameraPreview.takePicture({
+ *   maxWidth: 640,
+ *   maxHeight: 640
+ * });
+ *
+ * // Switch camera
+ * CameraPreview.switchCamera();
+ *
+ * // set color effect to negative
+ * CameraPreview.setColorEffect('negative');
+ *
+ * // Stop the camera preview
+ * CameraPreview.stopCamera();
+ *
+ * ```
+ *
  */
 @Plugin({
   plugin: 'cordova-plugin-camera-preview',
@@ -42,9 +88,7 @@ export class CameraPreview {
   @Cordova({
     sync: true
   })
-  static startCamera(rect: CameraPreviewRect, defaultCamera: string, tapEnabled: boolean, dragEnabled: boolean, toBack: boolean, alpha: number): void {
-
-  };
+  static startCamera(rect: CameraPreviewRect, defaultCamera: string, tapEnabled: boolean, dragEnabled: boolean, toBack: boolean, alpha: number): void { };
 
   /**
    * Stops the camera preview instance.