Add IAM signer (#108)

diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index 2d50672..244d0bb 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -24,5 +24,6 @@
    google.auth.crypt
    google.auth.environment_vars
    google.auth.exceptions
+   google.auth.iam
    google.auth.jwt