Document process for updating Schema

[Vidushi-GitHub]

Description

For Unified Schema we need to set-up rules for:
-getting approval from producers team for new version
-Tag the version: More rigid rules and examples for Major, Minor changes
-Announcement for New notice types

Acceptance criteria

• Add examples to the release instructions in the README file: new mission is a minor change, breaking change to an existing schema is a major change
• add this info to producer doc page