Add README with quick start example

Add a README.md that gives developers a quick understanding of what
Aether is and how to get started. Includes:
- Project description and why Aether exists
- Installation instructions
- Quick start code example showing event creation, persistence, and replay
- Key concepts (immutability, derived state, version consistency)
- Links to further documentation
- CI badge

Closes #44

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

README.md

@@ -0,0 +1,141 @@
+# Aether
+
+[![CI](https://git.flowmade.one/flowmade-one/aether/actions/workflows/ci.yml/badge.svg)](https://git.flowmade.one/flowmade-one/aether/actions/workflows/ci.yml)
+
+Event sourcing primitives for Go, powered by NATS.
+
+Aether provides composable building blocks for distributed, event-sourced systems without imposing framework opinions on your domain.
+
+## Why Aether?
+
+Building distributed, event-sourced systems in Go requires assembling many pieces: event storage, pub/sub, clustering, leader election. Existing solutions are either too heavy (full frameworks with opinions about your domain), too light (just pub/sub), or not NATS-native.
+
+Aether provides clear primitives that compose well:
+
+- **Event sourcing primitives** - Event, EventStore interface, snapshots
+- **Event stores** - In-memory (testing) and JetStream (production)
+- **Event bus** - Local and NATS-backed pub/sub with namespace isolation
+- **Cluster management** - Node discovery, leader election, shard distribution
+
+Built for JetStream from the ground up, not bolted on.
+
+## Installation
+
+\`\`\`bash
+go get git.flowmade.one/flowmade-one/aether
+\`\`\`
+
+Requires Go 1.23 or later.
+
+## Quick Start
+
+Here is a minimal example showing event sourcing fundamentals: creating events, saving them to a store, and replaying to rebuild state.
+
+\`\`\`go
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/google/uuid"
+	"git.flowmade.one/flowmade-one/aether"
+	"git.flowmade.one/flowmade-one/aether/store"
+)
+
+func main() {
+	// Create an in-memory event store (use JetStream for production)
+	eventStore := store.NewInMemoryEventStore()
+
+	// Create and save events
+	orderID := "order-123"
+
+	orderPlaced := &aether.Event{
+		ID:        uuid.New().String(),
+		EventType: "OrderPlaced",
+		ActorID:   orderID,
+		Version:   1,
+		Data:      map[string]interface{}{"total": 99.99, "items": 3},
+		Timestamp: time.Now(),
+	}
+	eventStore.SaveEvent(orderPlaced)
+
+	orderShipped := &aether.Event{
+		ID:        uuid.New().String(),
+		EventType: "OrderShipped",
+		ActorID:   orderID,
+		Version:   2,
+		Data:      map[string]interface{}{"carrier": "FastShip", "tracking": "FS123456"},
+		Timestamp: time.Now(),
+	}
+	eventStore.SaveEvent(orderShipped)
+
+	// Replay events to rebuild state
+	events, _ := eventStore.GetEvents(orderID, 0)
+
+	state := make(map[string]interface{})
+	for _, event := range events {
+		switch event.EventType {
+		case "OrderPlaced":
+			state["total"] = event.Data["total"]
+			state["items"] = event.Data["items"]
+			state["status"] = "placed"
+		case "OrderShipped":
+			state["status"] = "shipped"
+			state["carrier"] = event.Data["carrier"]
+			state["tracking"] = event.Data["tracking"]
+		}
+	}
+
+	fmt.Printf("Order state after replaying %d events:\n", len(events))
+	fmt.Printf("  Status: %s\n", state["status"])
+	fmt.Printf("  Total: $%.2f\n", state["total"])
+	fmt.Printf("  Tracking: %s\n", state["tracking"])
+}
+\`\`\`
+
+Output:
+\`\`\`
+Order state after replaying 2 events:
+  Status: shipped
+  Total: $99.99
+  Tracking: FS123456
+\`\`\`
+
+## Key Concepts
+
+### Events are immutable
+
+Events represent facts about what happened. Once saved, they are never modified - you only append new events.
+
+### State is derived
+
+Current state is always derived by replaying events. This gives you a complete audit trail and the ability to rebuild state at any point in time.
+
+### Versions ensure consistency
+
+Each event for an actor must have a strictly increasing version number. This enables optimistic concurrency control:
+
+\`\`\`go
+currentVersion, _ := eventStore.GetLatestVersion(actorID)
+
+event := &aether.Event{
+	ActorID: actorID,
+	Version: currentVersion + 1,
+	// ...
+}
+
+err := eventStore.SaveEvent(event)
+if errors.Is(err, aether.ErrVersionConflict) {
+	// Another writer saved first - reload and retry
+}
+\`\`\`
+
+## Documentation
+
+- [Vision](./vision.md) - Product vision and design principles
+- [CLAUDE.md](./CLAUDE.md) - Development guide and architecture details
+
+## License
+
+See [LICENSE](./LICENSE) for details.