docs: rewrite README for public release

README.md

@@ -1,16 +1,44 @@
 # cardano-checkout
 
-Merchant-side Cardano payment lifecycle in Python. Zero-custody.
+Merchant-side Cardano payment lifecycle for Python. Zero-custody.
 
-Ships the invoice state machine + UTxO watcher + reprice loop.
-Per-invoice HD-derived receive addresses, Koios polling, confirm /
-underpay / overpay classification, time-windowed repricing against
-your own oracle.
+`cardano-checkout` ships the invoice state machine, an on-chain UTxO
+watcher, and a quote-reprice loop for accepting ADA payments at
+per-invoice, HD-derived receive addresses. It polls [Koios](https://koios.rest)
+for payment, classifies each invoice as confirmed / underpaid / overpaid
+within a tolerance, and reprices expired quotes against a price oracle you
+supply.
 
-Does NOT ship Cardano primitives. Address derivation, chain context,
-transaction building, native-script minting, signing — use
-[pycardano](https://github.com/Python-Cardano/pycardano) directly.
-This library slots next to it.
+It does **not** reimplement Cardano primitives. Address derivation, chain
+context, transaction building, native-script minting, and signing are all
+[pycardano](https://github.com/Python-Cardano/pycardano)'s job — this
+library slots in next to it and owns only the merchant payment lifecycle.
+
+## Why
+
+Accepting on-chain payments for a shop is mostly bookkeeping, not
+cryptography:
+
+- one fresh receive address per order (derived from your wallet xpub),
+- watch the chain until the expected amount lands,
+- decide confirmed / underpaid / overpaid within a tolerance,
+- if the quote window lapses before payment, reprice and try again.
+
+`cardano-checkout` is that bookkeeping, behind small consumer-supplied
+interfaces — a persistence `Protocol` and a pricing callable — so it drops
+into any stack: SQLAlchemy, asyncpg, SQLite, or plain in-memory.
+
+## Install
+
+```
+pip install cardano-checkout                  # core
+pip install 'cardano-checkout[sqlalchemy]'    # + SQLAlchemy extra
+```
+
+Two runtime dependencies: `httpx` (Koios HTTP) and `apscheduler`
+(background loop). There is **no** `pycardano` dependency — see
+[Deriving addresses with pycardano](#deriving-addresses-with-pycardano)
+for the pairing pattern.
 
 ## Quick start
 
@@ -25,7 +53,7 @@ from cardano_checkout import (
 
 # Your oracle. Anything async returning int lovelace works.
 async def my_price_fn(usd: float) -> int:
-    rate = await fetch_ada_usd_somewhere()   # CoinGecko, Koios, fixed rate, etc.
+    rate = await fetch_ada_usd_somewhere()   # CoinGecko, a DEX, a fixed rate, etc.
     return int(round(usd / rate * 1_000_000))
 
 
@@ -53,8 +81,15 @@ async def main() -> None:
 asyncio.run(main())
 ```
 
+If you price your invoices in fixed ADA you can omit `price_fn` — the
+reprice job becomes a no-op and invoices simply expire at `expires_at`.
+
 ## Deriving addresses with pycardano
 
+The receive address for each invoice is derived from your **account-level
+xpub** (a public key — not a secret). Customer funds flow directly to your
+wallet; this library never touches keys.
+
 ```python
 from pycardano import HDWallet, Address, Network
 
@@ -78,7 +113,10 @@ addr = derive_address(account, index=42)
 
 ## NFT cert: CIP-25 v2 metadata
 
-Copy-paste builder for an on-chain cert per paid order. No dep.
+Need a certificate-of-authenticity NFT per paid order? Here is a
+dependency-free builder for the CIP-25 v2 metadata envelope. Hand the
+result to pycardano's `AuxiliaryData(Metadata({...}))` when you build the
+mint transaction.
 
 ```python
 def build_cip25_metadata(
@@ -119,12 +157,9 @@ def build_cip25_metadata(
     }
 ```
 
-Hand the dict to pycardano's `AuxiliaryData(Metadata({...}))` when
-building the mint tx.
-
 ## Implementing your own InvoiceStore
 
-`InvoiceStore` is a Protocol — implement six methods against whatever
+`InvoiceStore` is a `Protocol` — implement six methods against whatever
 backend you want (SQLAlchemy, asyncpg, SQLite, in-memory).
 
 ```python
@@ -139,7 +174,8 @@ class MySqliteStore:
     async def record_tx(self, invoice_id: str, tx_hash: str, lovelace_delta: int) -> None: ...
 ```
 
-See `InMemoryStore` in `cardano_checkout/store.py` for a reference impl.
+See `InMemoryStore` in `cardano_checkout/store.py` for a reference
+implementation (it also backs the test suite).
 
 ## Modules
 
@@ -150,18 +186,34 @@ See `InMemoryStore` in `cardano_checkout/store.py` for a reference impl.
 | `monitor.py` | `check_address_utxos` (Koios), `evaluate_utxos`, `check_pending_invoices`, `reprice_expired_invoices` |
 | `scheduler.py` | `InvoiceScheduler` — APScheduler wrapper, 15s check + 60s reprice |
 
-Two direct deps: `httpx`, `apscheduler`. No pycardano dep.
-
 ## Design
 
-1. **Protocol-first.** Persistence, pricing, side-effects through
-   consumer-supplied interfaces.
-2. **Use pycardano directly.** No wrapping of primitives.
+1. **Protocol-first.** Persistence, pricing, and side-effects all go
+   through consumer-supplied interfaces.
+2. **Use pycardano directly.** No wrapping of Cardano primitives.
 3. **Zero-custody.** Merchant keys never touch this code. xpub-derived
    addresses, UTxO observation, state transitions. Funds flow directly
    between customer and merchant wallets.
-4. **Offline-first tests.** Koios + price oracles stubbed via fixture.
+4. **Offline-first tests.** Koios and price oracles are stubbed via
+   fixtures — the suite never touches a live node.
+
+## Contributing
+
+Issues and pull requests are welcome. A couple of house rules keep the
+library focused:
+
+- **Keep Cardano primitives out.** Anything pycardano already does
+  belongs in the consumer, not here.
+- **Tests stay offline.** Koios and any price oracle must be stubbed via
+  fixtures so CI never hits a live node or a real wallet.
+
+Run the suite before opening a PR:
+
+```
+pip install -e '.[test]'
+pytest
+```
 
 ## License
 
-Apache-2.0.
+Apache-2.0. See [LICENSE](LICENSE).