Add PR checklist to contributing docs

[gwgundersen]

I'm a new contributor working on a couple PRs for Xarray and wished I had a short checklist of things to do before pushing a commit. First, I think such an overview list is useful per se. Second, CI takes a while, and it's much faster to just run black . or to build the docs manually than it is easy wait for CI to fail.

Inspired by Scikit-learn's checklist. This is just a first draft; happy for suggestions or to add anything I'm missing.

[max-sixty]

Great!! Thanks @gwgundersen

I added one note; do you want to also add to whatnew and give yourself credit?

We could even replace some of the instructions above with this; I find them overly verbose for a quick reference and not good enough as a complete reference. But maybe for another day

[gwgundersen]

I did not want to overreach with this PR, but I agree that the documentation is verbose. It seems to follow a logical order of information without making reasonable assumptions about the contributor. I think the docs would be better suited following an inverted pyramid of information. In that light, something like this PR checklist or an overview would be at the top with quick links to more detailed information at the bottom.

For example, while it's true that you need to know about git before contributing, how many people need to learn git or forking or creating branches when first contributing? My guess is that this is the minority, and the docs could reflect this by putting such reference material near the bottom of the page.

Also added a note to "What's new".

[max-sixty]

For example, while it's true that you need to know about git before contributing, how many people need to learn git or forking or creating branches when first contributing? My guess is that this is the minority, and the docs could reflect this by putting such reference material near the bottom of the page.

💯

[max-sixty]

Thanks again @gwgundersen