docs: versioning policy

[jemgillam]

Add to policy: Breaking changes to particular parts of dev tooling eg ruru instrumentation don't need a major semver bump

[benjie]

Essentially I plan to change the way that explain works, and I'm likely to add to Ruru's functionality over time. The bundled version of Ruru should always work with that version of PostGraphile, but what we're saying is that if we change the format of the configuration or response for the instrumentation payloads then this won't count as a breaking change since it's development only - these features aren't intended for production usage.

[benjie]

This might have been at least partially addressed already? @jemgillam to investigate.