Specify and document versioning policy

[cratelyn]

We need to document how we version our artifacts. Namely:

• viceroy-lib, the library providing functionality to run Compute@Edge binaries.
• viceroy-cli, the binary that is installed by users, which provides a daemon to run these programs locally.
• The relationship between these two; i.e. which -lib versions can be used by which -cli versions, and what constitutes a breaking change for each.