Need to explain how to get "Self" in RestAPI section of the Specification

[sumps]

There is no mention of the following RestAPI URL, which I thought we wanted to make mandatory so that App developers who only use RestAPI can easily access the URN (MMSI) of the vessel...

http:///signalk/v1/api/self

We need to make sure this is explicitly stated in the RestAPI page of the specification. We can also clarify what the response should be as raised in Issue #518