Build fails due to lack of GraalVM

[karianna]

When executing mvn client:build I get:

[ERROR] Failed to execute goal com.gluonhq:client-maven-plugin:0.1.27:compile (default-cli) on project devtools: Error: GraalVM installation directory not found. Either set GRAALVM_HOME as an environment variable or set graalvmHome in client-plugin configuration -> [Help 1]

easy enough to fix I guess but README may need updating.