Bluetooth: IPSP: Add README

This removes the section explaining how to test the IPSP from the general
README moving it ipsp specific README which can be expanded to include
other specific details related to 6LoWPAN, etc.

Change-Id: I80210cacb6b0042b3b0dc570dbdf7d38f92b3031
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>