README.md: add a note about ROS_OS_OVERRIDE
README.md: add a note about ROS_OS_OVERRIDE

meta-ros is supposed to be used for building a custom distribution
which isn't derived from any of the OSs known to the rospkg.os_detect
module.

Signed-off-by: Dmitry Rozhkov <208d90a35521dcb3e50d907c695d85f9e609147c@linux.intel.com>

rapyuta/meta-ros,bmwcarit/meta-ros,bmwcarit/meta-ros,rapyuta/meta-ros,rapyuta/meta-ros,bmwcarit/meta-ros,bulwahn/meta-ros,bmwcarit/meta-ros,bulwahn/meta-ros,bulwahn/meta-ros