Add user guide for Java / Kotlin project

[alexander-yevsyukov]

https://github.com/SpineEventEngine/embed-code-go/blob/master/showcase/README.md

This is the complete usage guide for embed-code-go. It is also executable: the showcase test runs the examples through the real CLI so the guide stays in sync with the application.

The Showcase is the guide for Go developers. It is a good start for new contributors, but it does not work for people who want to use the tool from projects in other languages.

Being a Java / Kotlin programmer I don't want to learn Go to find out how to set up embed-code for my project.
I want simple step by step instructions for my Gradle project. Let's forget about Maven for now.

Let's start from instructions for Java / Kotlin based on Gradle. Then we'll need to provide similar guides for other popular programming languages.

[alexander-yevsyukov]

@Oleg-Melnik, please pick up TypeScript as a separate documentation target. We'll need it for validation-ts and other TypeScript-based projects really soon.

[alexander-yevsyukov]

@Oleg-Melnik, if you'll see that you need a small Gradle plugin for embed-code-go to work smoothly with Gradle projects, don't be shy. We know how to write Gradle plugins.