docs(readme): innersource notice

[10xLaCroixDrinker]

Description

Added notice of innersourcing to README

Motivation

Prep for archival

Test Conditions

N/A

Types of changes

Check boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (adding or updating documentation)
• Dependency update
• Security update

Checklist

Check boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation and I have updated the documentation accordingly.
• These changes should be applied to a maintenance branch.
• I have added the Apache 2.0 license header to any new files created.