Highlight increased Java requirement

The usage of `java.awt.Toolkit.getMenuShortcutKeyMaskEx()` is
only available in Java 10+.

README.md

@@ -4,7 +4,7 @@ Cyber-physical systems simulation software following a Holon-based smart grid sy
 
 ## Prerequisites
 
-* Java 1.8 compatible VM
+* Java 10 compatible VM
 
 ## Introduction and User Manual