Guidance for marking an API as a prototype

[Ryandaydev]

Add guidance for methods that can be used to mark an API as a prototype, non-production API.

A few methods we have implemented with the City Pairs Prototype:

• API documentation includes "Prototype" in the title.
• API documentation includes the following statement "It is not a production API, and it is not intended for use by the public."
• API version is "V0"
• Response object contains a message "This is a prototype API. These results are for demonstration purposes only." in the payload.