[ci skip] Add J17 note to README as well (#748)

README.md

@@ -48,6 +48,8 @@ Building
 After cloning this repository, build the project with Gradle by running `./gradlew build` and take the created jar out
 of the `build/libs` directory.
 
+You need JDK 17 or newer to build ViaBackwards.
+
 License
 -
 This project is licensed under the [GNU General Public License Version 3](LICENSE).