More README files

- More details in the root README.md file
- Add a lot of README files in subdirectories

Signed-off-by: David Wagner <david.wagner@intel.com>
Concerning XSD files, shall we really describe all their content here. Will that be maintainable ? Isn't XSD file content enough to understand how to write files ? If not, wouldn't it be easier to comment into XSD file directly ?