feat: add GOOGLE_API_USE_CLIENT_CERTIFICATE support (#592)

diff --git a/docs/reference/google.auth.transport.mtls.rst b/docs/reference/google.auth.transport.mtls.rst
new file mode 100644
index 0000000..11b50e2
--- /dev/null
+++ b/docs/reference/google.auth.transport.mtls.rst
@@ -0,0 +1,7 @@
+google.auth.transport.mtls module
+=================================
+
+.. automodule:: google.auth.transport.mtls
+   :members:
+   :inherited-members:
+   :show-inheritance: