Add "JDK must include JavaFX" requirement

Author: skuzmich

README.md

@@ -203,7 +203,7 @@ to Gradle (in Preferences -> Build, Execution, Deployment -> Build Tools -> Grad
 
 ### Requirements
 
-* JDK >= 1.8 referred to by the `JAVA_HOME` environment variable.
+* JDK >= 1.8 referred to by the `JAVA_HOME` environment variable. JDK must include JavaFX.
 * JDK 1.6 referred to by the `JDK_16` environment variable. It is okay to have `JDK_16` pointing to `JAVA_HOME` for external contributions.
 
 ## Contributions and releases