Add acknowledgment to ClimateMachine.jl repository in Documentation / README.

[jakebolewski]

We should add an acknowledgment to the original ClimateMachine.jl repository (eventually paper with DOI link) and explain the respective differences between ClimateMachine and ClimaCore for users.

[valeriabarra]

I agree. There are many ways in which we can acknowledge recent and past contributions, and contributors.
We don't have to wait for a paper to have a DOI. For instance, GitHub has a nice integration with Zenodo DOIs. We can also add a CITATION.cff file so that we can have a "Cite this repository" button on our landing repository GitHub page (see attached example).

Also, related to this, I've tried to advocate for a "Project Alumni" section on our project website, even just a list of alumni names, nothin fancy.

[jakebolewski]

One of the long term admins is probably the best to add Zendo integration for the organization. Zendo also needs a release IIRC.

[valeriabarra]

Yes, and ClimateMachine.jl has two releases (since we are talking about ways to cite ClimateMachine.jl and acknowledge its contributors)

[simonbyrne]

We do have Zenodo hooked up now:
https://zenodo.org/record/5748473

[valeriabarra]

Ok, I started two quick PRs, one for ClimateMachine (PR #2250) and one for ClimaCore (PR #329 ) to add Zenodo's DOI badges to the top-level README for now. If we want to add the "Cite this repository" button as well I can look into adding the CITATION.cff file