docs: add contributing guide for Soroban smart contracts

[BigJohn-dev]

Closes #185

Description

Adds contracts/CONTRIBUTING.md with Soroban-specific guidance that the root CONTRIBUTING.md doesn't cover.

Problem: Root CONTRIBUTING.md only references generic Conventional Commits — no guidance on how to add a new entry point, when to bump an error enum, where to add tests, or how to handle storage migrations.

Solution: A dedicated contracts/CONTRIBUTING.md covering the full smart-contract workflow for Stellar/Soroban contributors.

Changes

Section	What it covers
Adding a New Entry Point	Step-by-step checklist (Design → Implement → Test → Submit)
Adding a New Contract	Directory creation, Cargo.toml template, workspace registration in Cargo.toml
Error Enum Discipline	#[repr(u32)] sequential discriminants (start at 1, never reorder/remove, always append), common variants table
Events Taxonomy	Topic schema (domain / action / optional entity id), symbol_short! rules (≤9 chars), data payload conventions, stable topic requirement
Testing	Inline #[cfg(test)] mod tests pattern, setup_test_env() helper, #[should_panic(expected = "Error(Contract, #N)")] conventions with discriminant comments, CI pipeline reference
Data-Model Migrations	Full u128 → struct upgrade walkthrough: deploy new contract, write temp migrate entry point, run on testnet, remove after confirmation
Audit Checklist	30+ items across: correctness (auth, overflow checks), storage (DataKey discipline), events (every mutation emits), errors (full coverage with #[should_panic] tests), WASM build, deployment
Quick Reference	Command table for all common operations

How to test

cd contracts
cargo test --workspace
Checklist
Documentation updated
Relevant issue linked
Changelog updated (N/A — docs-only change)

[snowrugar-beep]

LGTM