Some newcomers sometimes browse gitea or GitHub to learn about
OpenStack and naturally open the openstack/openstack repository,
only to find a confusing and pretty technical explanation about
submodule tracking and the subtleties of tested combinations of
components.

Provide some general information about OpenStack first, to
redirect those lose souls to friendler shores.

Change-Id: I41cde655f6c5d08fc482c40953687570acd59d3f