Update README

[DJViking]

The README should be updated with more information.

Problems I encounted when trying this plugin out with my JavaFX application.

Execution failed for task ':nativeRunAgent'.

GraalVM installation directory not found. Either set GRAALVM_HOME as an environment variable or set graalvmHome in the client-plugin configuration

The README should mention that you need GraalVM on your computer, and need to set the GRAALVM_HOME environment variable.

Which GraalVM do you need: The GraalVM CE build or Gluon GraalVM build?

Execution failed for task ':nativeRunAgent'.

Error creating Substrate Dispatcher: java.lang.NullPointerException: Main class name is required

Execution failed for task ':nativeCompile'.

Main class name is required

Where do I need to specify the class name?

When looking at a maven project, I see the property main.class is defined in pom.xml
<main.class>com.mycompany.sample.Main</main.class>

Setting main.class in my gradle.properties did not work
main.class = com.mycompany.sample.Main

Setting main.class on the command line to gradle did not work
./gradlew -P main.class=com.mycompany.sample.Main nativeBuild

Some searching, I found how to get it to work in #127
Set this in build.gradle
mainClassName = 'no.spacetec.qlv.QuicklookApplicationLauncher'

[mipastgt]

For point 1:
The Readme has a topic Requirements which referes you to the target platform where the requirement is explicitly stated together with a download link of the right version.