Add documentation for new prop

Author: jakewtaylor

README.md

@@ -76,6 +76,9 @@ import 'react-simple-hook-modal/dist/styles.css';
 
 - `backdropClassName` which can contain one or more classes to append and override the default styles (e.g. Changing the backdrop colour can be done by adding the class `bg-blue-800`).
 
+`Modal` also takes optional props:
+- `modalClassName` which can contain one or more classes to append to the default modal div.
+
 # Example
 
 See the `example` directory in the repository for a full example including multiple stacked modals.