Provide documentation on image contents

[mthalman]

There should be a readme that provides clear documentation on what software is provided in the images, most importantly for the SDK images since they contain a variety of things. We shouldn't require that consumers locate and parse through a Dockerfile to have an understanding of what they're getting. As example, the contents of the VMs for Azure DevOps agents are nicely documented: https://github.com/microsoft/azure-pipelines-image-generation#image-contents.

[MichaelSimons]

Closing as the basic content is described in the readme which is available in GitHub and DockerHub. A good example is the SDK readme - https://hub.docker.com/_/microsoft-dotnet-framework-sdk/