update SUMMARY.md

Concepts/README.md

@@ -5,3 +5,4 @@ This chapter describes various concepts which are used in the Linux kernel.
 * [Per-CPU variables](per-cpu.md)
 * [CPU masks](cpumask.md)
 * [The initcall mechanism](initcall.md)
+* [Notification Chains](notification_chains.md)

SUMMARY.md

@@ -63,6 +63,7 @@
     * [Per-CPU variables](Concepts/per-cpu.md)
     * [Cpumasks](Concepts/cpumask.md)
     * [The initcall mechanism](Concepts/initcall.md)
+    * [Notification Chains](Concepts/notification_chains.md)
 * [Data Structures in the Linux Kernel](DataStructures/README.md)
     * [Doubly linked list](DataStructures/dlist.md)
     * [Radix tree](DataStructures/radix-tree.md)