Add "How to add parts to index" section in README.md

[alexanderilyin]

Related docs examples which I love:

• https://containers.dev/features
• https://containers.dev/implementors/features/
• https://containers.dev/collections
• https://github.com/devcontainers/feature-starter
• https://containers.dev/implementors/features/#referencing-a-feature
• https://github.com/devcontainers/devcontainers.github.io/blob/gh-pages/_data/collection-index.yml

Also GH has following feature which might be useful:

• https://docs.github.com/en/repositories/creating-and-managing-repositories/creating-a-template-repository