Update Ajv versions section of the README

[simonplend]

• Identify breaking changes between Ajv v6 and v8 which will affect common Ajv usage patterns
  • Migration notes: https://ajv.js.org/v6-to-v8-migration.html
• Add 'Upgrading from v2 to v3' section
  • Make special note about need to use ajv-formats