Implement application default credentials (#32)

diff --git a/docs/reference/google.auth.environment_vars.rst b/docs/reference/google.auth.environment_vars.rst
new file mode 100644
index 0000000..fe34849
--- /dev/null
+++ b/docs/reference/google.auth.environment_vars.rst
@@ -0,0 +1,7 @@
+google.auth.environment_vars module
+===================================
+
+.. automodule:: google.auth.environment_vars
+    :members:
+    :inherited-members:
+    :show-inheritance: