Improve README files

[bwateratmsft]

All of the README files in this repo need some attention.

• Root: https://github.com/microsoft/vscode-docker-extensibility/blob/main/README.md
• container-runtimes: https://github.com/microsoft/vscode-docker-extensibility/blob/main/packages/container-runtimes/README.md
• vscode-container-runtimes: https://github.com/microsoft/vscode-docker-extensibility/blob/main/packages/vscode-container-runtimes/README.md
• vscode-docker-registries: https://github.com/microsoft/vscode-docker-extensibility/blob/main/packages/vscode-docker-registries/README.md
• CONTRIBUTING.md file