dns: FIX add README_API file and improve sample code

README_API file is added in this patch.

Inline code documentation is improved. More test domains are added.
The README file is also updated to reflect these changes.

Change-Id: Ie670a6559611c6a8d216470e245dbea48369696f
Signed-off-by: Flavio Santes <flavio.santes@intel.com>