Add creating/deleting instructions to Spanner README (GoogleCloudPlatform#920)

Author: bshaffer

spanner/README.md

@@ -49,6 +49,15 @@ authentication:
 [additional_scopes]: https://cloud.google.com/compute/docs/authentication#using
 [service_account_key_file]: https://developers.google.com/identity/protocols/OAuth2ServiceAccount#creatinganaccount
 
+### Create an instance
+
+These samples require you to first set up a [Spanner Instance][create-instance].
+Once you've finished with the samples, you can [delete your instance][delete-instance]
+to prevent incuring any additional charges.
+
+[create-instance]: https://cloud.google.com/spanner/docs/create-manage-instances
+[delete-instance]: https://cloud.google.com/spanner/docs/create-manage-instances#delete-instance
+
 ## Install Dependencies
 
 1. Ensure the [gRPC PHP Extension][php_grpc] is installed and enabled on your machine.