Take the little bit of documentation for resources from various commit
messages and combine it into a new devlink-resource.rst file.

This could probably be expanded on even further by someone with more
knowledge of how the devlink resources work.
Signed-off-by: NJacob Keller <jacob.e.keller@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>