docs: add step-by-step contract deployment guide

docs/deployment.md

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+# Contract Deployment Guide
+
+## Prerequisites
+
+- Rust + `rustup` (stable toolchain)
+- `stellar-cli`: `cargo install --locked stellar-cli --features opt`
+- WASM target: `rustup target add wasm32v1-none`
+- A funded Stellar account (source keypair)
+
+## Environment Variables
+
+| Variable | Description |
+|---|---|
+| `STELLAR_SECRET_KEY` | Deployer keypair secret |
+| `STELLAR_NETWORK` | `testnet` or `mainnet` |
+| `STELLAR_RPC_URL` | Soroban RPC endpoint |
+
+## Testnet Deployment
+
+```bash
+make setup && make build
+stellar contract deploy   --wasm target/wasm32v1-none/release/campaign.wasm   --source $STELLAR_SECRET_KEY   --network testnet
+```
+
+## Mainnet Deployment
+
+```bash
+stellar contract deploy   --wasm target/wasm32v1-none/release/campaign.wasm   --source $STELLAR_SECRET_KEY   --network mainnet   --rpc-url $STELLAR_RPC_URL
+```
+
+## Contract Initialization
+
+```bash
+stellar contract invoke   --id $CONTRACT_ID   --source $STELLAR_SECRET_KEY   --network testnet   -- initialize
+```
+
+## Troubleshooting
+
+- **`InsufficientFee`**: Add `--fee 1000000` to the deploy command.
+- **`WasmAlreadyExists`**: Binary is already on-chain; proceed directly to `invoke`.
+- **WASM target missing**: Run `rustup target add wasm32v1-none`.