docs: update README with API documentation and usage examples (#15)

* docs: update README with API endpoints, configuration, and architecture

* docs: update token flow diagram to show integration endpoint

* docs: fix integration token expiry to 20 seconds

* docs: simplify token flow diagram

* docs: add integration endpoint usage with request/response examples

* docs: address review feedback

Author: robertodauria

README.md

@@ -6,3 +6,136 @@
 [![Go Reference](https://pkg.go.dev/badge/github.com/m-lab/token-exchange.svg)](https://pkg.go.dev/github.com/m-lab/token-exchange)
 
 A Cloud Run service that exchanges API keys for signed JWTs for M-Lab services.
+
+## API Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `POST` | `/v0/token/autojoin` | Exchange autojoin API key for JWT token |
+| `POST` | `/v0/token/integration` | Exchange client integration API key for JWT token |
+| `GET` | `/.well-known/jwks.json` | Serve JSON Web Key Set for token verification |
+| `GET` | `/health` | Health check endpoint |
+
+## Development
+
+### Build and Test
+
+```bash
+# Build all packages
+go build -v ./...
+
+# Run all tests with race detection
+go test -race -v ./...
+
+# Run tests with coverage
+go test -coverprofile=coverage.out -coverpkg=./... ./...
+```
+
+### Docker
+
+```bash
+# Build Docker image
+docker build -t token-exchange .
+```
+
+### Deploy to Cloud Run
+
+```bash
+gcloud builds submit --config cloudbuild.yaml
+```
+
+## Configuration
+
+Configuration is via command-line flags. All flags can also be set via environment variables (e.g., `-port` becomes `PORT`, `-platform-ns` becomes `PLATFORM_NS`).
+
+| Flag | Default | Description |
+|------|---------|-------------|
+| `-port` | `8080` | Port to listen on |
+| `-private-key-path` | `/secrets/jwk-priv.json` | Path to JWK private key file |
+| `-platform-ns` | `platform-credentials` | Datastore namespace for autojoin credentials |
+| `-client-integration-ns` | `client-integration` | Datastore namespace for client integration credentials |
+| `-project-id` | `mlab-sandbox` | Google Cloud project ID |
+
+## CLI Tools
+
+### create-integration
+
+Creates a new client integration and API key in Datastore.
+
+```bash
+go run ./cmd/create-integration -project=mlab-sandbox -integration-id=my-integration
+```
+
+| Flag | Default | Description |
+|------|---------|-------------|
+| `-project` | (required) | Google Cloud project ID |
+| `-integration-id` | (required) | Integration ID |
+| `-namespace` | `client-integration` | Datastore namespace |
+| `-key-id` | (auto-generated) | Key ID |
+| `-integration-description` | | Human-readable description for the integration |
+| `-key-description` | | Human-readable description for the API key |
+| `-key-tier` | `0` | Service tier for the API key |
+
+## Usage
+
+### Integration Token Exchange
+
+Exchange an API key for a short-lived JWT token.
+
+**Request:**
+
+```bash
+curl -X POST https://your-service-url/v0/token/integration \
+  -H "Content-Type: application/json" \
+  -d '{"api_key": "your-api-key"}'
+```
+
+**Response:**
+
+```json
+{
+  "token": "eyJhbGciOiJSUzI1NiIsImtpZCI6Ii..."
+}
+```
+
+**JWT Claims:**
+
+| Claim | Description |
+|-------|-------------|
+| `int_id` | Integration ID |
+| `key_id` | API Key ID |
+| `tier` | Service tier |
+| `aud` | Audience (`integration`) |
+| `exp` | Expiration time (20 seconds from issue) |
+| `iat` | Issued at time |
+| `iss` | Issuer |
+| `jti` | Unique token ID |
+| `nbf` | Not valid before time |
+
+**Error Responses:**
+
+| Status | Description |
+|--------|-------------|
+| `400 Bad Request` | Invalid request body |
+| `401 Unauthorized` | Invalid API key |
+| `405 Method Not Allowed` | Request method is not POST |
+| `500 Internal Server Error` | Failed to generate token |
+
+## Architecture
+
+The service validates API keys stored in Google Cloud Datastore and returns signed JWT tokens. Tokens are signed using RS256 with keys loaded from a JWK file.
+
+### Token Flow
+
+```mermaid
+sequenceDiagram
+    participant Client
+    participant TokenExchange
+    participant Datastore
+
+    Client->>TokenExchange: POST /v0/token/integration (API key)
+    TokenExchange->>Datastore: Validate API key
+    Datastore-->>TokenExchange: Integration info
+    TokenExchange->>TokenExchange: Generate JWT (20-second expiry)
+    TokenExchange-->>Client: Signed JWT token
+```