Add some usage documentation

[Joacchim]

Currently, there is no proper documentation about how to use the tool:

• What parameters are mandatory ? -> source, destination, cloudmig/buckets
• What can help to explain an issue ? -> Assumptions of the different configuration variables (ie: cloudmig/buckets assumes the / ending for a directory, otherwise the names are just prepended)
  etc...

Also some basic configurations might be useful, even a simple configuration file with all the default values set in; it would be almost like launching cloudmig with the minimal number of parameters, but everything would be written explicitly,