Document application/json vs application/x-www-form-urlencoded

[waxlamp]

Add documentation about how Tangelo behaves when a content type of application/json is specified, vs. specifying application/x-www-form-urlencoded or not explicitly specifying it. In the former, any data attached to the request will be transmitted in the body of the request, while in the latter cases, it will instead be transmitted in the query arguments.

See #495 for motivating issue report.

(cc @kaneplusplus)