Docs: add example of how to set EXIF GPS metadata

lib/output.js

@@ -177,12 +177,18 @@ function toBuffer (options, callback) {
  *   .then(info => { ... });
  *
  * @example
- * // Set "IFD0-Copyright" in output EXIF metadata
+ * // Set output EXIF metadata
  * const data = await sharp(input)
  *   .withMetadata({
  *     exif: {
  *       IFD0: {
- *         Copyright: 'Wernham Hogg'
+ *         Copyright: 'The National Gallery'
+ *       },
+ *       IFD3: {
+ *         GPSLatitudeRef: 'N',
+ *         GPSLatitude: '51/1 30/1 3230/100',
+ *         GPSLongitudeRef: 'W',
+ *         GPSLongitude: '0/1 7/1 4366/100'
  *       }
  *     }
  *   })