docs(reth): add Base Sepolia network settings to README

[Bashmunta]

Added explicit Testnet values (RETH_CHAIN=base-sepolia, OP_NODE_NETWORK=base-sepolia, Sequencer https://sepolia-sequencer.base.org) to align with Quick Start’s .env.sepolia and prevent misconfiguration. This clarifies correct identifiers for Base L2 Sepolia across clients.

[cb-heimdall]

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