diff --git a/README.rst b/README.rst
index da31a119..2f7c33cf 100644
--- a/README.rst
+++ b/README.rst
@@ -216,3 +216,10 @@ Name, version and release of the module.
 Specifies the exact repository state from which a module is built.
 
 E.g. ``"scmurl": "git://pkgs.stg.fedoraproject.org/modules/testmodule.git?#020ea37251df5019fde9e7899d2f7d7a987dfbf5"``
+
+``topdir``
+~~~~~~~~~~
+
+The toplevel directory containing the trees for each architecture of a module.
+This field is only present when a module finished building, i.e. with the
+states 'done' or 'ready'.