Add API documentation for External Sources

[JosephVolosin]

Add API documentation for creating External Sources/Events - this is tied to pull-request #198 and the related pull-requests in aerie/aerie-ui/aerie-gateway. The documentation should cover the use of the two new Gateway end-points: createExternalSourceEventTypes and createExternalSource.

[JosephVolosin]

Included in PR #198 , see this commit