Replace README.md with instructions for contributors.

[gheber]

Let's use the README.md file to document the DOs and DON'Ts for this project. Let's also tell people how to test the changes they've made before creating a PR.

[loricooperhdf]

@gheber See a first stab of this at https://github.com/loricooperhdf/HDFGroup.github.io/blob/master/README.md

Not at all sure if this is what you were looking for--how do you want them to test changes? Also since this duplicated content of index.md I want to confirm that this won't change the text on the homepage.

[lkurz]

Also since this duplicated content of index.md I want to confirm that this won't change the text on the homepage.

The README.md is for the REPO. The index.md is for the SITE homepage.
They are independent of each other, except in the case when there is no index.md file, then github-pages uses the README.md, but the SUPPORT (ie non-github-pages) site would have an empty body for the homepage.

Set up GitHub pages on your fork to test your changes before submitting.

Instead developers can also work from Githb Codespaces or use a container with (or without) VSCode on a local dev machine. More details to come pertaining to this scenario.

[lkurz]

Lori, can you PR this to a branch on GHIO repo?
Also can we remove all the completed PR branches to clean up.