Added link to table of contents to all notebooks.

this is so the user of nbviewer can navigate the book more easily.

Chapter04_Gaussians/Gaussians.ipynb

@@ -1,13 +1,20 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:03a16f585bb3f4a954520990ebb17e6cf9ceb7f9e247874263f66bfe4ac0da5a"
+  "signature": "sha256:c6d0174467636b2460a28523c35141b3937e125a51ec438d4a4184f4c898666b"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
  "worksheets": [
   {
    "cells": [
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "http://nbviewer.ipython.org/github/rlabbe/Kalman-and-Bayesian-Filters-in-Python/blob/master/table_of_contents.ipynb"
+     ]
+    },
     {
      "cell_type": "heading",
      "level": 1,