Checklist for 2.1.0 release

[ha0ye]

With the updated docs and new features, I think it'd be timely to think about what else to take care of for a 2.1.0 release. (since 2.0.0 is now approaching a year on CRAN, and it seems like there's been some substantial under-the-hood fixes and added functionality)

[ethanwhite]

Agreed. I think the only question is the version. We're getting ready to release retriever 3.0 and there are changes here that require retriever 3.0, so I think we probably want to make this 3.0.0.

[ethanwhite]

I don't see any blockers in the issue queue, but if anyone sees anything that needs to go in before the next release we should go ahead and identify those now.