Automate the list of upcoming events in `README.md`

[gr2m]

This is a follow up to #278.

@anguspiv mentioned that one pain point

Making it easier to discover what locations and times the next event is coming

We added upcoming events to the README, but that will get out of sync pretty quickly. So my goal is to automate updates to that section using GitHub Actions, similar to what I did in gr2m/helpdesk#10