Update SUMMARY.md

SUMMARY.md

@@ -10,7 +10,8 @@
     * [First steps in the kernel](Initialization/linux-initialization-1.md)
     * [Early interrupts handler](Initialization/linux-initialization-2.md)
     * [Kernel entry point]()
-* [Data Structures in the Linux Kernel]()
+* [Data Structures in the Linux Kernel](DataStructures/README.md)
+    * [Doubly linked list](DataStructures/dlist.md)
 * [Theory](Theory/README.md)
     * [Paging](Theory/Paging.md)
     * [Elf64](Theory/ELF.md)