doc: add ABI guidelines

doc: add ABI guidelines

People have been asking for ways to use the ABI macros, heres some docs to
clarify their use.  Included is:

* An overview of what ABI is
* Details of the ABI deprecation process
* Details of the versioning macros
* Examples of their use
* Details of how to use the ABI validator

Thanks to John Mcnamara, who duplicated much of this effort at Intel while I was
working on it.  Much of the introductory material was gathered and cleaned up by
him.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>