doc Improve README

In case someone gets lost in the internet and navigates to our GH repo, she should see something more informative than general info and a couple of links.

IMO, README.md in the root of GH repo is kind of landing page that should be representative. This is a first step, we should perhaps also add a couple of images here.

@olaf-k @piuccio @divdavem can some of you have a look?
Preview: https://github.com/jakub-g/ariatemplates/tree/README - feedback and changes are welcome.