Add CONTRIBUTING.md

This filename is recognized by GitHub:

  https://help.github.com/articles/setting-guidelines-for-repository-contributors/



Point contributors at the spdk.io development page so they can learn how
to use Gerrit.

Change-Id: I155b9437a8e3b5490bf4ce982fe47918140f5268
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/376848


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>