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

diff --git a/docs/reference/google.auth.credentials.rst b/docs/reference/google.auth.credentials.rst
new file mode 100644
index 0000000..646fa7b
--- /dev/null
+++ b/docs/reference/google.auth.credentials.rst
@@ -0,0 +1,7 @@
+google.auth.credentials module
+==============================
+
+.. automodule:: google.auth.credentials
+    :members:
+    :undoc-members:
+    :show-inheritance: