docs(audience): add UPM-visible README for the Audience package (SDK-150)

Adds src/Packages/Audience/README.md so studios clicking the package
in Unity Package Manager see install instructions, a working
first-event snippet, and a link to the full integration guide on
docs.immutable.com.

Author: ImmutableJeffrey

src/Packages/Audience/README.md

@@ -0,0 +1,51 @@
+# Immutable Audience
+
+Typed C# tracking SDK for Unity games. Captures `game_launch`, `session_start` / `session_heartbeat` / `session_end` automatically; predefined events (`Progression`, `Resource`, `Purchase`, `MilestoneReached`) and custom events on demand.
+
+> **Status:** alpha. APIs and behavior may change between releases.
+
+## Install
+
+In Unity, open **Window → Package Manager**, click **+ → Add package from git URL...**, and paste:
+
+```
+https://github.com/immutable/unity-immutable-sdk.git?path=src/Packages/Audience#main
+```
+
+Requires Unity 2021.3 or later. Works under Mono and IL2CPP.
+
+## First event
+
+```csharp
+using Immutable.Audience;
+using UnityEngine;
+
+public static class Analytics
+{
+    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
+    private static void Init()
+    {
+        ImmutableAudience.Init(new AudienceConfig
+        {
+            PublishableKey = "YOUR_PUBLISHABLE_KEY",
+            Consent = ConsentLevel.Anonymous,
+            DistributionPlatform = DistributionPlatforms.Steam,
+            Debug = true,
+        });
+
+        ImmutableAudience.Track(new Purchase { Currency = "USD", Value = 9.99m });
+    }
+}
+```
+
+Press Play; `ImmutableAudience.Initialized` returns `true` and `AnonymousId` becomes a non-null GUID. The SDK warns to the Unity Console with prefix `[ImmutableAudience]` only on errors.
+
+## Documentation
+
+The full integration guide, API reference, event schemas, and troubleshooting live at:
+
+- https://docs.immutable.com/docs/products/audience/unity-sdk
+
+## License
+
+See the repository [LICENSE](https://github.com/immutable/unity-immutable-sdk/blob/main/LICENSE.md).