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.
mkcloud L483 has this, but maybe someone dropped the init script in 13.1? Then we could still use chkconfig which is LSB standard and should thus work everywhere.