docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to YieldLadder
+
+Thank you for your interest in contributing to YieldLadder! This guide covers the project structure and local development workflows.
+
+## Project Structure
+
+YieldLadder is organised as a monorepo with three main components:
+
+- **Soroban Contracts** (Rust): Cargo workspace in `contracts/`
+- **Next.js Dashboard** (TypeScript): Web app in `app/`
+- **TypeScript SDK**: Client library in `sdks/typescript/`
+
+## Prerequisites
+
+- Rust toolchain with `wasm32-unknown-unknown` target
+- Node.js 20+ and pnpm 9+
+- Soroban CLI (for contract deployment and testing)
+
+## Running Locally
+
+### Soroban Contracts
+
+The contracts are organised as a Cargo workspace. To build all contracts:
+
+```bash
+cargo build --target wasm32-unknown-unknown --release
+```
+
+Run tests:
+
+```bash
+cargo test
+```
+
+### Next.js Dashboard
+
+Navigate to the `app/` directory:
+
+```bash
+cd app
+pnpm install
+pnpm dev
+```
+
+The dashboard will be available at `http://localhost:3000`.
+
+Run typechecking:
+
+```bash
+pnpm typecheck
+```
+
+### TypeScript SDK
+
+Navigate to the `sdks/typescript/` directory:
+
+```bash
+cd sdks/typescript
+pnpm install
+pnpm build
+```
+
+Run typechecking:
+
+```bash
+pnpm typecheck
+```
+
+## Submitting Changes
+
+1. Fork the repository
+2. Create a feature branch from `main`
+3. Make your changes and ensure all CI checks pass
+4. Submit a pull request with a clear description
+
+All pull requests must pass CI checks including contract builds, Next.js typechecking, and SDK typechecking.