README: add info about develop & test branches

include repo command lines and basic explanation of master and
master-next along with caveats.

Change-Id: I60d1a5dd625b0abb0bb442c4b0df5bfde08c1ed5
Signed-off-by: Christian Betz <christian.betz@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>