Add Samples section to README.md

**What**

Add a **Samples** section to `README.md` explaining how to import and run the UPM package sample that ships with the SDK.

The package includes one sample registered in `package.json`:

- **Display name:** `Open Creator Rails Sample`
- **Path:** `Samples~/Open Creator Rails`
- **Contents:** Two demo scenes (`Loading.unity`, `Demo.unity`), UI toolkit layouts, and gameplay scripts demonstrating gated access via `GatedAssetTrigger` / `GatedAssetSubscriptionHandler`

The section should cover:

1. **Importing the sample** — open **Window → Package Manager**, find *Open Creator Rails Unity*, go to the **Samples** tab, and click **Import** next to *Open Creator Rails Sample*.
2. **What gets imported** — briefly describe what lands in `Assets/Samples/`: the two scenes, UI assets, and key scripts.
3. **Running the demo** — open the `Loading` scene and hit Play; what the demo shows (subscription-gated area, subscribe flow, access-granted state).
4. **Prerequisites** — the scene requires a running indexer, a funded wallet in `StreamingAssets/secrets.json`, and the `OpenCreatorRailsService` configured per the existing Setup section.

**Why**

The package already ships a complete working sample but the README has no mention of it. Users who install the package have no indication that a demo exists or how to access it through the Package Manager UI.

**How**

1. Add a `## Samples` section to `README.md` after the existing **Installation** section (or after **Setup** — wherever it reads most naturally).
2. Use a short numbered list for the import steps.
3. Keep it concise; link back to the Setup section for wallet/service configuration rather than repeating it.

**Acceptance Criteria**

- [ ] `README.md` contains a `## Samples` (or equivalent) section.
- [ ] Import steps (Package Manager → Samples tab → Import) are documented.
- [ ] The section describes what the sample demonstrates.
- [ ] Prerequisites point back to the existing Setup section rather than duplicating content.

**Estimation**

**Dependencies**

None.