Adding reference to Azure CNI as 3rd party plugin in readme
Adding reference to Azure CNI as 3rd party plugin in readme

Signed-off-by: Chandan Aggarwal <8b9550237f863c2851828c4adb9180917742ac8a@microsoft.com>

appc/cni,containernetworking/cni,containernetworking/cni,appc/cni