Add migration notice to design-tokens repo

[georgewrmarshall]

Description

Add a migration notice to the top of the repository README to inform developers that this package is currently being migrated to the metamask-design-system monorepo. This notice helps to prevent any unintended commits or PRs in the source repository during the migration phase, directing active development to the new monorepo location.

Technical Details

• Place the migration notice in a table at the very top of the README.
• Ensure that the message is clearly visible and links directly to the metamask-design-system monorepo.
• The notice should advise developers to avoid commits in the source repository and to re-open active PRs in the new repository.

The migration notice content:

<table><tr><td><p align="center"><b>⚠️ PLEASE READ ⚠️</b></p><p align="center">This package is currently being migrated to our <a href="https://github.com/MetaMask/metamask-design-system"><code>metamask-design-system</code></a> monorepo. Please do not make any commits to this repository while this migration is taking place, as they will not be transferred over. Also, please re-open PRs that are under active development in the metamask module template.</p></td></tr></table>

• Example for reference: eth-json-rpc-provider Migration PR

Acceptance Criteria

• Migration notice is added to the top of the README.
• Notice links correctly to the metamask-design-system monorepo.
• Notice discourages further commits in this repository during migration.
• PRs under active development are advised to re-open in the new repository.

References

• Package Migration Process Guide