Add google.auth.credentials - the public interfaces for credentials (#8)

diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index ef0d689..78f7786 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -19,6 +19,7 @@
 
 .. toctree::
 
+   google.auth.credentials
    google.auth.crypt
    google.auth.exceptions
    google.auth.jwt