Callout need for `requirements.txt` for use in shinylive

[Garrick Aden-Buie]

The key example in the front of https://rstudio.github.io/py-shinyswatch/#usage doesn't work if you copy the code in app.py directly into shinylive.io. I think it's worth calling out the need for requirements.txt to use shinyswatch in shinylive. (Here's an example where a user wasn't aware of this need.)