Gradle: use implementation instead of compile (Docs)

[bertilmuth]

The use of compile to declare Gradle dependencies has been deprecated for a while and as far as I know even removed from Gradle 7.x
(See https://docs.gradle.org/current/userguide/upgrading_version_5.html)

So in the documentation, use implementation to declare the Gradle dependencies instead.

[hschwentner]

I updated the README and added a bit more info. @bertilmuth what do you think?

[bertilmuth]

Looks good!