Popolo `start_event` and `end_event` example.

start_event-end_event.json

// Old member
{
  "organization_id": "house-of-commons",
  "person_id": "john-doe",
  "end_event": "john-doe-resigns"
}

// Resignation
{
  "id": "john-doe-resigns",
  "description": "John Doe resigned to spend more time with family."
}

// New member
{
  "organization_id": "house-of-commons",
  "person_id": "jane-doe",
  "start_event": "jane-doe-substituted"
}

// Substitution
{
  "id": "jane-doe-substituted",
  "description": "Jane Doe is substituted for John Doe"
}