Update README.md (recommend suitable L1 RPC for withdrawals)

[MenyMarquez1978]

Summary

• Add a short note in the Usage section recommending L1 RPC endpoints that can serve historical blocks and suggesting archive-style providers for older withdrawals

Rationale

• Helps users avoid confusing errors when their L1 RPC cannot look up the withdrawal or dispute game due to limited history

[cb-heimdall]

🟡 Heimdall Review Status

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Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1