add "event" discovery

[tantek]

Since there are Micropub implementation(s) (https://micropub.net/implementation-reports/clients/) that support creating h-event content, document how to discover events / event posts (and if there is a distinction)

[tantek]

Event markup with h-event (http://microformats.org/wiki/h-event) has been both stable for a while and published by over a dozen implementations: https://indieweb.org/event#IndieWeb_Examples

And is consumed by 3+ per https://indieweb.org/event#Consuming_Tools_And_Services

Thus adding minimal detection accordingly purely for typing reasons based on "h-event" root class name.

[tantek]

Added with commit b6f3259