Add README to sample directory

FirebaseVertexAI/Sample/README.md

@@ -0,0 +1,16 @@
+# Vertex AI for Firebase Sample App
+
+You can try out the SDK quickly, see a complete implementation of various use
+cases, or use the sample app if you don't have your own Apple platforms app. To
+use the sample app, you'll need to perform the following steps:
+1. Download
+   [vertexai-preview-0.1.0.zip](https://github.com/firebase/firebase-ios-sdk/archive/refs/heads/vertexai-preview-0.1.0.zip)
+   for the latest release of the SDK
+1. Extract the zip and open `GenerativeAISample.xcodeproj` in the
+   `FirebaseVertexAI/Sample` directory
+1. [Register the sample app](https://firebase.google.com/docs/ios/setup#register-app)
+   in your Firebase project
+   - The default bundle ID is `com.google.firebase.VertexAISample`
+1. [Download the `GoogleService-Info.plist`](https://firebase.google.com/docs/ios/setup#add-config-file)
+   to the `FirebaseVertexAI/Sample` directory, overwriting the placeholder file
+   with the same name