Update README.md

Adds Greenfoot as a project that uses RichTextFX

Author: amjdhsn

README.md

@@ -50,6 +50,7 @@ Who uses RichTextFX?
 - [BlueJ](https://www.bluej.org/)
 - [JabRef](http://www.jabref.org/)
 - [PsychSQL](http://softbydoc.dx.am/?page_id=135)
+- [Greenfoot](https://www.greenfoot.org/)
 
 If you use RichTextFX in an interesting project, I would like to know!