Define a `CONTRIBUTING.md` for the smart-contract workflow

[snowrugar-beep]

• Problem Statement. CONTRIBUTING.md references generic
  Conventional Commits but no contract-specific guidance: how to
  add a new entry point, when to bump an error enum, where to add
  tests.
• Why it matters. Smart-contract contributors come from the
  Soroban/wagmi world; common Rust pattern but new to Stellar.
• Expected Outcome. contracts/CONTRIBUTING.md with: testing
  guidance (cargo test), event taxonomy guidance, migration
  steps (uint128 → struct change), audit checklist.
• Acceptance Criteria.
  1. PR adding a new entry point follows the checklist.
• Files / Modules. new contracts/CONTRIBUTING.md.
• Difficulty. Easy.
• Effort. 1 day.
• Labels. docs, contracts, community-onboarding.

[Heazzy500]

Hello, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[BigJohn-dev]

Allow me fix this pls

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #185)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@BigJohn-dev's PR #192 was approved and merged by @snowrugar-beep.

🏆 @BigJohn-dev: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,444 total points). Track your full progress on GrantFox.

👏 Great work, @BigJohn-dev! Keep contributing to VertexChainLabs.