Update build instructions in README after switch to Gradle

[dreis2211]

Hi,

the README still mentions mvn clean install and alike for building the project.

Cheers,
Christoph

[philwebb]

Thanks, we've got a few issues like this left to sort out.

[wilkinsona]

Thanks, @dreis2211. I remembered the notes in CONTRIBUTING.adoc but forgot that the README had some too.