Document package addition checklist

Add comprehensive checklist for adding new packages to the monorepo.
This helps ensure all necessary files are updated when introducing
new packages, preventing common oversights.

The checklist is divided into:

- Required updates (always needed)
- Conditional updates (based on package type)
- Optional updates (as needed)

[ci skip]

Co-Authored-By: Claude <noreply@anthropic.com>