docs: add project status + pilot caveat to README

[stern-shawn]

I don't have a cool JIRA link to paste here 😢

@garethpaul had requested we add a clear caveat about the pilot status of the repo while I get up to speed on this. Lmk if Insiders isn't appropriate to reference here.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [n/a] I have added tests that prove my fix is effective or that my feature works
• [n/a] I have added the necessary documentation about the functionality in the appropriate .md file
• [n/a] I have added inline documentation to the code I modified

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