Add instructions for using pyref.dev as a custom search engine

[treyhunner]

I think it's worth noting the idea and the fact that https://pyref.dev/%s would be the most preferable endpoint URL to use, as users might not realize this could save them even more time.

Comprehensive instructions might not be worth writing, but maybe it's worth linking to instructions elsewhere.

[treyhunner]

Using https://pyref.dev/is?symbol=%s or even https://pyref.dev/is?lucky=1&symbol=%s might be useful as well.