Update README.md with CodeCommit de-emphasis notification

[ViLNguyen]

Description of your changes

The Guidance that links to this repo will include a notification regarding CodeCommit. Adding the same notification to the README will ensure the notice is clear to customers who did not come from the Guidance page.

How to verify this change

Related issues, RFCs

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• [ x] I have performed a self-review of my own code
• [ x] I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• [ x] I have made corresponding changes to the documentation (e.g. README.md)
• [ x] My changes generate no new warnings

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.