doc: add readme for pitr

[mlycore]

Type of change:

• [ ] Bugfix
• [ ] New feature provided
• [ ] Improve performance
• [ ] Backport patches
• [x] Docs

What this PR does / why we need it:

Pre-submission checklist:

• [ ] Did you explain what problem does this PR solve? Or what new features have been added?
• [ ] Have you added corresponding test cases?
• [ ] Have you modified the corresponding document?

[sonarcloud[bot]]

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