Docs: add example of how to set EXIF GPS metadata

docs/api-output.md

@@ -140,12 +140,18 @@ sharp('input.jpg')
 ```
 **Example**  
 ```js
-// 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'
       }
     }
   })