Add example showing how targeted endpoints work

[trevorhardy]

Targeted endpoints are nifty and we need an example showing what they do and how to use them.

Targeted endpoints can only send to endpoints that they target. If no specific endpoint is given as the destination then all targeted endpoints get the message. A specific endpoint can be the destination of the message but it must be in the provided list of targeted endpoints.

This provides a way to segment sets of endpoints such that only endpoints in a target group are dependent on each other. Normally, all endpoints are dependent on all other endpoints but making groups of targeted endpoints allows breaking the endpoints into isolated groups that are not dependent on each other.