Add service account credentials (#25)

* Add service account credentials

* Address review comments

* Fix lint

* Fix docstring

* Address review comments
diff --git a/docs/reference/google.oauth2.service_account.rst b/docs/reference/google.oauth2.service_account.rst
new file mode 100644
index 0000000..cc4e438
--- /dev/null
+++ b/docs/reference/google.oauth2.service_account.rst
@@ -0,0 +1,7 @@
+google.oauth2.service_account module
+====================================
+
+.. automodule:: google.oauth2.service_account
+    :members:
+    :inherited-members:
+    :show-inheritance: