docs(readme): add innersource notice

[10xLaCroixDrinker]

Description

Add innersource notice to README

Motivation and Context

Prep for archival

How Has This Been Tested?

N/A

Types of Changes

• 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

Checklist:

• My change requires a change to the documentation and I have updated the documentation accordingly.
• My changes are in sync with the code style of this project.
• There aren't any other open Pull Requests for the same issue/update.
• These changes should be applied to a maintenance branch.
• I have added the Apache 2.0 license header to any new files created.

What is the Impact to Developers Using Purgecss-Loader?

No new open source releases of purgecss-loader