Add config parameter descriptions into README.md

[kaiyan-sheng]

What does this PR do?

This PR is to add missing config parameter descriptions into README.md file.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

[elasticmachine]

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• Duration: 15 min 33 sec

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