DOC: Update dependency for to_markdown documentation
Adding to the documentation that tabulate is required for to_markdown to be executed

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

Thanks @eyaltrabelsi , but we already raise an import error if `tabulate` isn't installed

```
ImportError: Missing optional dependency 'tabulate'.  Use pip or conda to install tabulate.
```

, does this need to be added to the docstring? @MarcoGorelli In my opinion it should be part of the documentation as well, but if you think its unneeded we can close this PR :) 