Schemas/Readme: replace 'TuningMode' with 'TuningAllowed'

There is no such "TuningMode" attribute; it was a mistake and the attribute
described in this documentation corresponds to "TuningAllowed".

Signed-off-by: David Wagner <david.wagner@intel.com>
Schemas/Readme: more details for each type of configuration file

The scope of this readme is widened to document the hierarchy of configuration
files and details on the content of each of them.

Signed-off-by: David Wagner <david.wagner@intel.com>
Merge pull request #26 from 01org/schemas-readme-fixes

Schemas readme fixes