chore: update README.md

[DOH-PXC5303]

Add appropriate NA string in R & Python read examples

📑 Description

How to Test

✅ Checks

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• I have read and followed the contributing guidelines.
• I have checked my code for any issues.
• I have updated the documentation (if applicable).
• I have added tests to cover my changes (if applicable).
• All tests are passing (if applicable).
• My changes do not introduce any new warnings.
• I have run the code locally and verified the changes work as expected.

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