Docs: Add config instructions on JavaFX Maven project #568

Description

I have found that issue #18 and PR #27 enabled the action to work on JavaFX projects, but it seems that this part lacks some documentation.

This would be very helpful if README can have more explicit record on it.

Are you willing to submit a PR?

Unfortunately, I have not completely figure out how to make it work.

My understanding is, for a JavaFX project using Maven:

1. Add javafx-maven-plugin in pom.xml according to Getting Started with JavaFX
2. Config the ci file as follows:

distribution: 'zulu'
cache: maven
java-package: jdk+fx

3. Use mvn compile or mvn javafx:run to compile/run the project in CI

If this is correct and nothing was missing, I'm glad to help.

Additionally, I'm not sure why using other distribusion of jdk cannot work, even maven can download JavaFX dependencies automatically.