clarify interface compatibility wrt semver

[Tamas K. Papp]

• no deprecations, API just changes with major versions, minor versions make upward-compatible changes

• encourage users to put a [compat] section in their code

• what is part of the API? anything exported from DynamicHMC, recursively. so ATM Diagnostics isn't strictly speaking a part of the API one can rely on. make this clear