docs(storage): add comment in doc on how to use STORAGE_EMULATOR_HOST (…

…#4656)

storage/doc.go

@@ -48,6 +48,31 @@ an unauthenticated client with
 
     client, err := storage.NewClient(ctx, option.WithoutAuthentication())
 
+To use an emulator with this library, you can set the STORAGE_EMULATOR_HOST
+environment variable to the address at which your emulator is running. This will
+send requests to that address instead of to Cloud Storage. You can then create
+and use a client as usual:
+
+    // Set STORAGE_EMULATOR_HOST environment variable.
+    err := os.Setenv("STORAGE_EMULATOR_HOST", "localhost:9000")
+    if err != nil {
+        // TODO: Handle error.
+    }
+
+    // Create client as usual.
+    client, err := storage.NewClient(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+
+    // This request is now directed to http://localhost:9000/storage/v1/b
+    // instead of https://storage.googleapis.com/storage/v1/b
+    if err := client.Bucket("my-bucket").Create(ctx, projectID, nil); err != nil {
+        // TODO: Handle error.
+    }
+
+Please note that there is no official emulator for Cloud Storage.
+
 Buckets
 
 A Google Cloud Storage bucket is a collection of objects. To work with a