README: expand README file with information for new users

This change adds the following sections to the README file:

* Installation, steps needed to prepare the host in order to use the
  automation scripts
* Configutation, brief description of the most important configuration
  keys and pointing out the complete list is available using `mkcloud -h`.
* Usage, example usage to create a cloud and run tempest.
* Contributing, a short description how to contribute to this repository.

This change expands the README file to include a section about the
'installation' of the automation scripts and steps needed to prepare
the host in order to use the automation scripts.