add an ical feed

[aaronj1335]

so that @caseysoftware can add it to atxtechevents 😁

[stedman]

Not sure if ical feed is still desired, but now we can add it via npm! https://www.npmjs.com/package/ical-generator

[stedman]

I also see https://www.addevent.com/add-to-calendar-button being used around quite a bit.