Improve Getting Started in the docs

[voddan]

Now instructions for starting using Cordentity are just gradle clean buil. Please make a usable guide!

[osesov]

Getting Started section should be written with a newbie in mind, and should describe at least two things:

1. How to run the tests and examples and verify if they work
2. References to code, which could be used as example of various indy+corda operations. That is the code to copy-and-paste into your project. Few words along with references to ReadmeExampleTest.kt and CordentityE2E.kt should be enough.