Add JaCoCo build and run documentation to README.

[BukunmiOla]

This addresses issue 518.

This allows contributors to have an express information on:

• How to get JaCoCo running with the project.
• How to generate test coverage reports with JaCoCo.
• And where to view the test coverage report.

[wbrunette]

@BukunmiOla I am not sure this is needed as it should get the gradle config from the remote without checking out gradle config from GitHub. Can you revise to be simply how to run the JaCoCo? Happy to discuss more.

[BukunmiOla]

Yes @wbrunette I figured that out later. I couldn't even build with it in directory. So, I'm going to update the PR like you suggested.