[CORD-1806] Document XOS internals

Change-Id: I790ec1c97acc94f215b4ed03d56ca201452cb3b7
(cherry picked from commit 804030bbfc6edde599875f5a716d173a56e0d51f)
diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 6b18a3c..65f9e22 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -16,6 +16,7 @@
     * [Build System Config Glossary](build_glossary.md)
 * [Operating and Managing CORD](operate/README.md)
     * [Powering Up a POD](operate/power_up.md)
+    * [XOS Internals](xos/xos_internals.md)
     * [REST API](operate/rest_apis.md)
     * [TOSCA](xos-tosca/README.md)
 * [Defining Models in CORD](xos/README.md)