Clarify that the JDK is required at runtime

README.md

@@ -10,7 +10,10 @@ released in late 2009.
 
 ## Prerequisites
 
-Building OpenRS2 requires version 8 or later of the [Java Development Kit][jdk].
+OpenRS2 requires version 8 or later of the [Java Development Kit][jdk].
+
+The JDK is required even if a pre-built copy of OpenRS2 is used, as it depends
+on JDK-only tools, such as `jarsigner`, at runtime.
 
 ### Non-free components