[T054] Create command developer guide

[petehauge]

Phase: Phase 7: US5 Extensibility
User Story: US5 - Extensible Command Architecture

Create command developer guide in src/cli/README.md

Document:

• How to add a new command
• File naming conventions
• Interface shape
• Option reuse
• Testing pattern

[petehauge]

This already exists in the readme: https://github.com/Azure/apiops-cli/blob/main/README.md . Closing!