Document how to determine a recurring event's ID

[kylecombes]

The means by which rec_id is determined needs to be documented somewhere. Currently I (the front end) am unable to request data for a specific recurrence of a recurring event.

Old frontend code suggests that it should be possible to request data for a specific recurrence of a recurring event series by making a GET request of the form /events/<event_sid>/<ms_since_unix_epoc_of_recurrence>. However, submitting GET /events/5adfc5a48b17bd000cbb6b86/1524614451000 to the prod server results in

{ "message": "The requested URL was not found on the server.  If you entered the URL manually please check your spelling and try again." }

@newsch also suggested I try querying using the ISO 8601 date format, but that (yyyy-mm-dd) results in the same response.

[kylecombes]

Actually, it'd probably be preferable for the server to generate recId and include it on each recurrence object.

[kylecombes]

It appears that the ISO 8601 format of YYYY-MM-DD works.