docs: add test harness usage to README

Document the local development test harness including:
- Prerequisites (Docker, yq, jq)
- Quick start commands (make up/down)
- Configuration options (cluster.yaml)
- Example topologies
- Available make targets
- Troubleshooting tips

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: bseto

README.md

@@ -118,6 +118,112 @@ $ ./_build/kvctl migrate slot 123 --target 1 -n test-ns -c test-cluster
 
 For the HTTP API, you can find the [HTTP API(work in progress)](docs/API.md) for more details.
 
+## Local Development Test Harness
+
+A config-driven test harness for quickly spinning up KVRocks clusters with any topology. Useful for integration testing, debugging, and local development.
+
+### Prerequisites
+
+- Docker and Docker Compose
+- [yq](https://github.com/mikefarah/yq) (YAML processor) - `brew install yq`
+- [jq](https://jqlang.github.io/jq/) (JSON processor) - `brew install jq`
+
+### Quick Start
+
+```bash
+# Start a cluster with default config (3 shards, 1 replica per shard)
+make up
+
+# Verify the cluster is ready
+redis-cli -p 7770 CLUSTER NODES
+redis-cli -p 7770 CLUSTER INFO
+
+# Tear down the cluster
+make down
+```
+
+### Configuration
+
+Edit `harness/config/cluster.yaml` to customize your cluster topology:
+
+```yaml
+# Number of shards (hash slots distributed across these)
+shards: 3
+
+# Replicas per shard (0 = primary only, 1 = primary + 1 replica, etc.)
+replicas_per_shard: 1
+
+# Base port for KVRocks nodes (allocates sequentially: 7770, 7771, ...)
+base_port: 7770
+
+# Cluster name (used in kvrocks-controller)
+name: test-cluster
+
+# Namespace (for kvrocks-controller organization)
+namespace: default
+```
+
+### Examples
+
+**Single-shard cluster with 2 replicas (1 primary + 2 replicas):**
+```yaml
+shards: 1
+replicas_per_shard: 2
+```
+
+**Multi-shard cluster for testing slot migration:**
+```yaml
+shards: 3
+replicas_per_shard: 1
+```
+
+**Minimal cluster for quick tests:**
+```yaml
+shards: 1
+replicas_per_shard: 0
+```
+
+### Available Commands
+
+| Command | Description |
+|---------|-------------|
+| `make up` | Start the cluster (generates compose, starts services, initializes cluster) |
+| `make down` | Stop and remove all containers and volumes |
+| `make generate` | Regenerate docker-compose.yml from config |
+| `make validate-config` | Validate cluster.yaml configuration |
+
+### Architecture
+
+The test harness creates:
+- **Consul** - Storage backend for kvrocks-controller
+- **kvrocks-controller** - Cluster management (built from source)
+- **KVRocks nodes** - One container per node with proper isolation
+
+All services have health checks, and `make up` blocks until everything is healthy and the cluster is initialized.
+
+### Troubleshooting
+
+**Check service status:**
+```bash
+docker compose -p kvrocks-harness -f harness/generated/docker-compose.yml ps
+```
+
+**View controller logs:**
+```bash
+docker logs kvrocks-controller
+```
+
+**View KVRocks node logs:**
+```bash
+docker logs kvrocks-0-0
+```
+
+**Connect to a specific node:**
+```bash
+redis-cli -p 7770  # First node
+redis-cli -p 7771  # Second node
+```
+
 ## License
 
 Licensed under the [Apache License, Version 2.0](LICENSE)