[flags-sdk] Add README for statsig example (#1091)

### Description

This PR adds a README to the statsig example

### Demo URL

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Provide a URL to a live deployment where we can test your PR. If a demo
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### Type of Change

- [ ] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

---------

Co-authored-by: Dominik Ferber <[email protected]>

flags-sdk/experimentation-statsig/README.md

@@ -0,0 +1,69 @@
+# Statsig Flags SDK Example
+
+This example uses [Statsig](https://vercel.com/marketplace/statsig) for feature flags with the [Flags SDK](https://flags-sdk.dev) along with the `@flags-sdk/statsig` [Statsig adapter](https://flags-sdk.dev/docs/api-reference/adapters/statsig) and the [Flags Explorer](https://vercel.com/docs/workflow-collaboration/feature-flags/using-vercel-toolbar).
+
+## Demo
+
+[https://flags-sdk-statsig.vercel.app/](https://flags-sdk-statsig.vercel.app/)
+
+## How it works
+
+This demo uses two feature gates on Statsig to control the visibility of two banners on the page.
+Both gates are configured to show/hide each banner 50% of the time.
+
+Once you visit the page, you can see a variation of both/one/none of the banners.
+Since this example is using a stable id to identify users, you will see the same variation all the time.
+
+To test different variations, you can use the Dev Tools at the bottom to reset the stable id and reload the page.
+This allows you to test different variations of the banners.
+
+If you deployed your own and configured the feature gates on Statsig, you can also use the [Flags Explorer](https://vercel.com/docs/workflow-collaboration/feature-flags/using-vercel-toolbar) to test different variations by creating overrides.
+
+## Deploy this template
+
+The easiest way to get started with Statsig is through the native integration in the [Vercel Marketplace](https://vercel.com/marketplace/statsig).
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/clone?repository-url=https://github.com/vercel/examples/tree/main/flags-sdk/experimentation-statsig&env=FLAGS_SECRET&envDescription=The FLAGS_SECRET will be used by the Flags Explorer to securely overwrite feature flags. Must be 32 random bytes, base64-encoded. Use the generated value or set your own.&envLink=https://vercel.com/docs/workflow-collaboration/feature-flags/supporting-feature-flags#flags_secret-environment-variable&project-name=statsig-flags-sdk-example&repository-name=statsig-flags-sdk-example&products=[{\"integrationSlug\":\"statsig\",\"productSlug\":\"statsig\",\"type\":\"integration\",\"protocol\":\"experimentation\"}])
+
+### Step 1: Link the project
+
+In order to use the Flags Explorer, you need to link the project on your local machine.
+
+```bash
+vercel link
+```
+
+Select the project from the list you just deployed.
+
+### Step 2: Pull all environment variables
+
+This allows the Flags SDK and the Flags Explorer to work correctly, by getting additional metadata.
+
+```bash
+vercel env pull
+```
+
+### Step 3: Create feature flags on Statsig
+
+Head over to the [Statsig Console](console.statsig.com) and create the two feature flags under Feature Gates:
+
+- `Summer Sale` with the gate id `summer_sale`
+- `Free Shipping` with the gate id `free_delivery`
+
+You can also find the gate ids in the `flags.ts` file.
+
+### Step 4: Configure the Feature Gates
+
+Select the `Free Delivery` and `Free Shipping` feature gates and configure them on the Statsig Console.
+
+Create a new rule by clicking on "+ Add New Rule" and set the percentage to 50%.
+
+After that, click on "Save" at the bottom right corner.
+
+### Step 5 (optional): Set additional environment variables
+
+If you provide the `STATSIG_CONSOLE_API_KEY` and `STATSIG_PROJECT_ID` environment variables, the Flags Explorer will fetch additional metadata from the Statsig API.
+
+This will show the description (if set) and displays a link to the feature gate on the Statsig Console.
+
+You can find both values in the [Statsig Console](https://console.statsig.com/settings/project).