Fix doc for rate limiting keys

[tunguyennnnn]

Background

Update README for informing rate limit on RPC keys

Changes

To Do

Additional Deployment and/or Rollback Steps

Code Review

This Pull Request was thoroughly reviewed and meets all Code Review Standards pertaining to:

• [ ] Implementation - satisfied acceptance criteria
• [ ] Communication - notified engineers/stakeholders about impactful changes
• [ ] Error handling - handled, logged & alerted on errors
• [ ] Documentation - wrote readable code & commits, documented unintuitive code
• [ ] Testing - unit & E2E test coverage, tested in staging, DB migrations backwards compatible
• [ ] Security - sanitized inputs, vetted dependencies, validated/secured API requests, no committed secrets
• [ ] Performance - optimized expensive algorithms & database queries

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