extension: add README.md to extension

[lcwangchao]

What problem does this PR solve?

Issue Number: ref #38493

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[bb7133]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, hawkingrei

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• OWNERS [bb7133,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-14 09:56:46.490552896 +0000 UTC m=+4156604.077663041: ☑️ agreed by hawkingrei.
• 2023-11-15 07:10:00.037084214 +0000 UTC m=+4232997.624194360: ☑️ agreed by bb7133.