DOC: add mention of recommended dependencies in users guide
First-time contributor here!

Ian Ozsvald mentioned installing the optional dependencies for pandas in his talk at PyData Amsterdam 2020. Although these are mentioned in the Getting Started section, I though they should be mentioned in the Users Guide, which is where I typically go for pandas information. It seems like many users aren't aware that these dependencies are available to increase performance.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
