document Prism's versioning policy

[EdVinyard]

Document answers to the following questions:

• What Node version(s) does Prism support?
• How are do Prism versions align with the Node LTS cycle?
• What is Prism's versioning policy? When is the major version incremented?

Also consider:

• supporting an earlier or wider range of Node version, and delaying increments to the minimum (How much do those versions need to be aligned?)
• using exact versions in prism-cli's dependencies