Add example for polling with reswap and HttpResponseStopPolling

[fdemmer]

Thank you for another useful, little package for Django, Adam! ❤️

Clicking through the examples after reading the docs, I thought maybe a demo for a timed trigger and HTTP 286 (without swapping!) would be a good addition.

[adamchainz]

Thanks for the suggestion. Using a global variable isn’t safe, because it shares state between all “countdown” processes. After trying to refactor the demo not to use it, I realized we also don’t need reswap - no need for an iterator and StopIteration check when we can just use a list index and check if it’s the last. So I don’t think this demo provides much value, at least for general use. Probably better as a blog post, if you want to write it up. Thanks!