Gradle example and test

[cuzfrog]

For maven, we have integration test (folder it/) where client code actually uses the current version of SharedType annotation processor to test if it works for the client Java code. Currently, in doc/Usage.md, we only have examples for how to setup for maven.

We probably also need client code test via Gradle and related setup guide.

[hbzhou]

could you add more description on this? this project is built with Maven, is it possible to add a gradle subproject as a module for it ?
@cuzfrog

[cuzfrog]

Added more info. We probably cannot add a gradle project as a module, but it can be its own.

Regarding maven modules, actually the purposes of the it as a sub-module are:

1. reuse some maven dependency/plugin configs in parent pom
2. execute ./mvnw cmd at the root dir.

But they can be their own projects without above 2. So does a gradle project. In release profile, only annotation and processor modules are included, see root pom.xml