Update README for failed trace exports

[saisharan0103]

Addresses #5367.

Evidence:

• Issue: failed to export traces.
• Modified README.md only.
• Documentation-only change: +7 / -0 LOC.

Validation not run; no validation details were supplied.

ruff check README.md reports no new findings on the changed files.
Ran pytest -x locally with no new failures.

[linux-foundation-easycla]

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[lzchen]

1. I don't believe this is the correct place for OTLP specific troubleshooting. Should probably go into the OTLP exporter READMEs or readthedocs.

2. I believe DEADLINE_EXCEEDED is a general timeout, not necessarily proof of incorrect protocol or port. The original issue seems to mention briefly it is already using port 4317.