Update user-guide.rst (#337)

Add additional way to supply creds to client without relying on I/O
diff --git a/docs/user-guide.rst b/docs/user-guide.rst
index 6283662..d43163f 100644
--- a/docs/user-guide.rst
+++ b/docs/user-guide.rst
@@ -56,7 +56,7 @@
 A service account private key file can be used to obtain credentials for a
 service account. You can create a private key using the `Credentials page of the
 Google Cloud Console`_. Once you have a private key you can either obtain
-credentials one of two ways:
+credentials one of three ways:
 
 1. Set the ``GOOGLE_APPLICATION_CREDENTIALS`` environment variable to the full
    path to your service account private key file
@@ -81,6 +81,20 @@
         scoped_credentials = credentials.with_scopes(
             ['https://www.googleapis.com/auth/cloud-platform'])
 
+3. Use :meth:`service_account.Credentials.from_service_account_info
+   <google.oauth2.service_account.Credentials.from_service_account_info>`::
+
+        import json
+
+        from google.oauth2 import service_account
+
+        json_acct_info = json.loads(function_to_get_json_creds())
+        credentials = service_account.Credentials.from_service_account_info(
+            json_acct_info)
+
+        scoped_credentials = credentials.with_scopes(
+            ['https://www.googleapis.com/auth/cloud-platform'])
+
 .. warning:: Private keys must be kept secret. If you expose your private key it
     is recommended to revoke it immediately from the Google Cloud Console.