Instruction on how to run azure-xplat-cli with docker in the release notes of azure-xplat-cli and maybe integrate a Dockerfile there

Following discussion on PR #22

@ahmetalpbalkan you are right this repo maybe make sense.

However what I can report is this repo not been updated for a while (0.9.11, 0.9.12 and 0.9.13 are missing as for now); I feel like if azure-xplat-cli team would be responsible of updating this would be nice.

I think also having a Dockerfile in azure-xplat-cli to build the current version would be great, so we can clone the repo, build the image and run without having node on the host system.

For docker releases I mean to have something like `docker run -it microsoft/azure-cli:X.X.X` in the release notes.

What do you think about?
Thanks