DOC: section in indexing user guide to show use of np.where
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add a section in the user guide to illustrate the use of `np.where` and `np.select` to enlarge a dataframe conditionally.  The example is taken from SO, as mentioned in [this issue](https://github.com/MarcoGorelli/PyDataGlobal2020-sprint/issues/5#issuecomment-727241262).