Add note on required jdk version for compilation

README.md

@@ -64,6 +64,8 @@ Please note the [differences in licensing](#license).
 
 Building:
 --------
+You need JDK 9 or higher to build ViaVersion. The plugin works on Java 8, but needs to be compiled with a higher version for compatibility reasons.
+
 After cloning this repository, build the project with Gradle by running `/gradlew build` and take the created jar out of
 the `build/libs` directory.