Add release summary, some touch-ups

[max-sixty]

This PR:

• Proposes adding a Release Summary with a PR. This is something that maybe more people read than anything else we send, and yet it's the least reviewed text of anything I write; and I'm making guesses on what's important.
• Proposes pasting the release summary into the GH Release page; to the extent people follow xarray by following our releases, it's nice to have something there.
• Some touch-ups

[shoyer]

Looks great, thanks Max!

[keewis]

Looks good to me, too. I have a few suggestions, though:

[keewis]

I think it definitely makes sense to get a review of the summary so I'd remove this.

Instead, I'd like to suggest writing the summary as item 2, then opening a PR with the contributors, the summary and the whats-new.rst fixes (new item 4) as item 5 to get a chance for a review and once that is merged continue with the current item 4 (after syncing again).

So now we'd have:

1. sync branch
2. write summary
3. get the contributors (also: what should happen in the rare case that the one releasing is not included in that list?)
4. check whats-new.rst and the docs
5. open a PR to add items 2 to 4 to whats-new.rst
6. resync (should that be a new step or should that also be in item 5?)
7. continue with the old item 4

Does that make sense?

[max-sixty]

Updated based on @keewis good suggestions. I haven't updated the numbers until we agree it's correct. (Maybe there's some plugin to do this? Lucky we don't have a 100 item list...)

[dcherian]

Thanks @max-sixty