Move .github/CONTRIBUTING.md to CONTRIBUTING.md and move relevant README content there

The current README.md is primarily information intended for contributors to this website, while the current .github/CONTRIBUTING.md is essentially empty.

I think we're missing an opportunity to let people who randomly find this repo on GitHub or who want to perform some other task (e.g. searching for events/people/WG, look through the organizational history, etc) learn more about what US-RSE is and where to find out more about it, as well as a good place to recognize all the contributors. I think that's what should be in README.md.

I also propose that we move CONTRIBUTING.md from the GitHub specific ./.github/CONTRIBUTING.md to the standard CONTRIBUTING.md in the repo root, and then move all the contribution-specific information there. Apart from being a more GitHub-independent way of organizing this file, I think it will be easier for contributors to refer to the instructions in this way. GitHub also displays "Remember, contributions to this repository should follow its contributing guidelines." at the bottom of every issue, so having something useful to link it from there would be useful.

Compare README.md with .github/CONTRIBUTING.md, then organize the contributor-specific material in a new root CONTRIBUTING.md and keep README.md focused on the US-RSE project, its resources, history, and contributors. Verify that the contribution link points to CONTRIBUTING.md and that both documents are complete and readable.