doc: update README

[alainncls]

Action: Update documentation (#263)

Context / Rationale:

• Adding token should now be done via the Developer Hub
• Unclear descriptions of the createdAt and updatedAt fields for a token

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PR Checklist

• Verified and published the contract source
• Kept the token list in alphabetical order by token symbol
• Updated the updatedAt field (and createdAt if applicable)
• Updated the list version number according to the README instructions

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Note

Updates README to route token additions via Developer Hub and adds detailed maintainer docs for manual token list updates.

• Docs:
  • README:
    • Route token additions through the Linea Developer Hub and simplify instructions.
    • Minor copy/format tweaks (link wrapping, wording).
  • Development Guide (docs/development.md):
    • Add "Manually add a token" section with example JSON, field definitions, and token type explanations.
    • Clarify createdAt/updatedAt meanings.
    • Include contribution steps, validation workflow, and guidelines (ordering, version bump rules).

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