docs: add local development README

[Osraka]

Summary

• Add a README with local setup, configuration, development, build, and validation steps.
• Document the build-time TEMPO_NETWORK and TEMPO_RPC_URL settings used by the esbuild scripts.
• Note where chain config, tokenlists, and Tempo ABIs/addresses are sourced from in the codebase.

Validation

• npm test
• npm run build

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