CONTRIBUTING: Add a contributing document
CONTRIBUTING: Add a contributing document

This is copied from virtcontainers.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

chavafg/runtime,gorozco1/runtime,chavafg/runtime,gorozco1/runtime,clearcontainers/runtime,clearcontainers/runtime,devimc/runtime,devimc/runtime