ONEID-4689: Document OAuth2 client credentials auth in README

[matthewkmartin]

Summary

• Adds an Authentication section to README.md documenting OAuth2 client credentials auth, marked as the preferred method, alongside existing basic auth
• Documents auto token refresh / retry-on-401 behavior and manual token management via autoRefreshToken/tempAccessToken
• Notes that credentials should never be hard coded and shows loading them via environment variables

Test plan

• Docs-only change; no code paths affected
• Reviewed rendered README for formatting consistency with existing sections

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