docs: improve README with prerequisites, verification steps, and configuration details

[2TheMoom]

This PR improves the README for node operators with the following additions:

• Prerequisites section — Docker version requirements and verification commands so operators know exactly what's needed before starting
• Node verification steps — how to confirm the node is running correctly after startup including an eth_blockNumber health check
• Concrete storage example — clarifies the storage formula with real numbers (e.g. 500GB chain + 200GB snapshot = ~1.44TB)
• BASE_NODE_VERIFIER_L1_CONFS documentation — env variable introduced in v0.15.3 that was undocumented
• Upgrading section — step-by-step upgrade commands with a note for pruned node operators
• Troubleshooting section — three common issues with solutions

[cb-heimdall]

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[osr21]

Great additions — the eth_blockNumber health check is exactly the kind of practical detail that saves time on first setup.

One thing worth adding to the verification section (or a gotchas note): the public http://localhost:8545 endpoint you're calling for eth_blockNumber is fine for local node operators, but if you're building an app that routes external traffic to a Base node, the free public https://mainnet.base.org endpoint rate-limits under production load.

We hit this running BasePay, a gasless relay on Base Mainnet. A relay handler that makes 7–9 sequential RPC calls per swap (balance checks, quotes, TX submits, receipt polling) trips the rate limit under modest traffic — the errors look like smart-contract failures but are actually just HTTP 429s from the RPC endpoint.

The fix is using the Coinbase Node authenticated endpoint:

# Public — rate-limited, no auth required
https://mainnet.base.org

# Authenticated — higher limits, same infrastructure
https://api.developer.coinbase.com/rpc/v1/base/<YOUR_CDP_API_KEY>

In code:

const transport = http(
  process.env.CDP_API_KEY
    ? `https://api.developer.coinbase.com/rpc/v1/base/${process.env.CDP_API_KEY}`
    : "https://mainnet.base.org" // local dev fallback
);

If the README's verification section notes this distinction — even just a line that mainnet.base.org is a shared public endpoint with rate limits — it would save builders from chasing phantom contract bugs.