Document how to get query stats enabled for pgHero

[lambada]

The setup guide has you install postgres; and the mastodon install sets up pgHero; however it doesn't document how to enable Query Stats in pgHero, and the big 'Enable' button doesn't work, saying the user doesn't have permission to enable it even though it is available; despite following the instructions from the setup guide exactly.

It would be great if the docs did provide instructions on enabling it, or granting the postgres user permissions to do so, or even just a pointer to pgHero's docs on how to do it over at https://github.com/ankane/pghero/blob/master/guides/Query-Stats.md