Update README.md

Concepts/README.md

@@ -4,3 +4,4 @@ This chapter describes various concepts which are used in the Linux kernel.
 
 * [Per-CPU variables](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html)
 * [CPU masks](http://0xax.gitbooks.io/linux-insides/content/Concepts/cpumask.html)
+* [The initcall mechanism](https://0xax.gitbooks.io/linux-insides/content/Concepts/initcall.html)