Add google.auth.impersonated_credentials (#299)

diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index 244d0bb..bc6740b 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -25,5 +25,6 @@
    google.auth.environment_vars
    google.auth.exceptions
    google.auth.iam
+   google.auth.impersonated_credentials
    google.auth.jwt