Adding README document for self-contained tests

[selldinesh]

This PR contains the readme document for running tests in a self-contained manner using ixia-c.
As the first step towards the process we have tried sending 100 packets using scapy on the sonic dut ports which are connected in a back to back manner and documenting the steps done

[chrispsommers]

Hi, this is a good start, but there is not enough context. Some suggestions:

• make a new directory usecases/selfcontained-dut and rename this file usecases/selfcontained-dut/README.md. This is where it will live when we're all done; treat this as a draft. This is about a self-contained DUT testbed, not about just about back to back testing. Someone might even use it with snake testing, or internal loopbacks eventually.
• write it as if it were a step-by-step tutorial for demonstrating this use-case. Also, in the section https://github.com/selldinesh/SAI-Challenger/tree/main/usecases#readme, add something like "See selfcontained-dut for a tutorial" so it links to this README.
• Add a link to the diagram e.g. ![saic-physical-dut-sw-tgen-self-contained](../../../img/saic-physical-dut-sw-tgen-self-contained.svg or whatever it needs to be, so it's in your README for reference.
• Provide all the steps you performed to get this running, including all relevant Dockerfile etc. steps you took after all our discussions.

Consider: you get once chance to tell him what you need in order to get answer by the morning. What must you tell him in this initial communication? The e-mail should be a simple cover letter/request, the README should be self-contained. Don't worry that it is incomplete and has dead-ends in it, you can fix all that later. But an outsider (Andriy) has nothing to go on but this; what information does he need to get to the same place you did?

Hope this sheds some light on my approach.