Add some general guide about adding new library function and
a few guides for their usage outside of tests.

Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Acked-by: Katarzyna Piecielska <katarzyna.piecielska@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>