Improve documentation for engineers on how to make changes to the schema

This MR attempts to improve the documentation of the Secure Report Schemas such that it is clearer to engineers on how to make changes. This is based partially on the outcome of a Secure brainstorming session.

Changes include:

• Document recommendations about what MODEL.REVISION.ADDITION version to change when making schema changes
• Explicitly support additionalProperties
  • This was previously implicitly supported (i.e. was true by default)
  • Add documentation describing reasons why can be useful
  • Explain that additional properties are considered experimental
• Document schema release suggestions
• Document suggestions for how to make changes that are future proof

👀 README.md 👀