Improve README with some samples

[gvespucci]

Hi, I landed on this page looking for a way to run KotlinTest and TestNG tests together on the same codebase built with Maven. I understood that having a TestNG engine on the JUnit5 platform would do the job. Unfortunately, as I'm a rookie in JUnit5, I couldn't find how to configure this testng-junit5 in a Maven project. Could you provide the README with some configuration hints? Thank you very much.

[sormuras]

This sub project is still in a very early state, so any documentation will be outdated "soon".

Before writing an How-to, the testng-junit5 artifact has to be published somewhere. Not sure, if it already is available on some repository out there.

That said, there's an integration test using a basic Maven project to verify the TestNGine is loaded and running as expected at: https://github.com/testng-team/testng-junit5/tree/master/src/it/basic
Find some inspiration there.