Add overlayOpacity option to readme

Author: piascikj

README.md

@@ -332,6 +332,7 @@ introJs().onafterchange(function(targetElement) {
  - `showButtons`: Show introduction navigation buttons or not, `true` or `false`
  - `showBullets`: Show introduction bullets or not, `true` or `false`
  - `scrollToElement`: Auto scroll to highlighted element if it's outside of viewport, `true` or `false`
+ - `overlayOpacity`: Adjust the overlay opacity, `Number`
 
 See [setOption](https://github.com/usablica/intro.js/#introjssetoptionoption-value) to see an example.