Update SUMMARY.md

SUMMARY.md

@@ -23,6 +23,7 @@
     * [Interrupt handlers](interrupts/interrupts-3.md)
     * [Initialization of non-early interrupt gates](interrupts/interrupts-4.md)
     * [Implementation of some exception handlers](interrupts/interrupts-5.md)
+    * [Handling Non-Maskable interrupts](interrupts/interrupts-6.md)
 * [Memory management](mm/README.md)
     * [Memblock](mm/linux-mm-1.md)
     * [Fixmaps and ioremap](mm/linux-mm-2.md)