Add Arm work description for Container4NFV project

Add Arm work description for Container4NFV project

As we know, Arm is an active partipants of the Container4NFV project.
Arm has done a lot of work to improve container more widely used for NFV
, which includes various projects of OPNFV focused on container based NFV.
Arm highly recognized the meaning of Container4NFV project, and all Arm's
work would align with the direction of Container4NFV project at the most
extent now and in the future.
The document gives out Arm's existed work for Container4NFV and other
containerized VNF related project. The possible roadmap for Arm's future work
on Container4NFV is also given.

The document contains the following sections:
1. Architecture
2. Container Networking
3. Related Projects
4. the Current Status and Future Plan
5. Contacts

In addition to the format revising, other modifications:

1. Add an index.rst to generate the doc tree
2. Rename the file to container4nfv_on_arm.rst
3. Remove trailing whitespace again
4. Modified the image declartion in the rst

Change-Id: I5f6486ec08c96feb504a50951adab444f035bd53
Signed-off-by: Trevor Tao <trevor.tao@arm.com>