Document packages and other projects using `s2fft`

[matt-graham]

Suggestion from @jasonmcewen:

What about adding a list of codes that use s2fft to the readme/docs somewhere? ... It could be mutually beneficial since it would help to promote other codes that make use of s2fft, while also being useful for us to demonstrate community usage of s2fft.

For Python packages specifically we can somewhat automate this by looking for reverse dependencies on PyPI: https://www.wheelodex.org/projects/s2fft/rdepends/ though this doesn't capture packages not on PyPI and use in non-library code