Docs: Mention the fact that this generates PEP-658 compatible metdata as well

[Phil Elson]

It is cool that this produces a full index, including PEP-658 metadata! That is way more than just PEP-503, and you should probably highlight that in your docs.

[J. Nick Koston]

Thanks! More than happy to merge a PR to update the docs if you would be inclined.