fix: add special logging fields section in the doc

[cindy-peng]

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Fixes #690 🦕 This is to add a small section to explain the special fields in structured logging.

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml