README: fence the code blocks
README: fence the code blocks

Signed-off-by: Fathi Boudra <7cd6e55cc07a66bc71d7901a4cddfe6c22aab659@linaro.org>

ndechesne/qcom-oe-manifest,linaro-home/lhg-oe-manifests,96boards/oe-rpb-manifest