Add instructions to update collection-index.yml to CONTRIBUTING.md

Right now the only place with instructions on what to add to the collections-index.yml file and how to do it are at https://containers.dev/collections. I think that there should be a similar blurb added to the CONTRIBUTING.md file for this repository.

Sidenote: it currently seems like the contributing guide for this repo focuses very heavily on the specification itself, and not the docs website that makes up the bulk of this repo.

https://github.com/devcontainers/devcontainers.github.io/blob/4b6f855e772e8e6cfa4fc5fc38460e01fb01443a/contributing.md