Added link to table of contents to all notebooks.

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

Chapter02_Discrete_Bayes/discrete_bayes.ipynb

@@ -1,13 +1,20 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:ec42c8b0a8ccc622dae96f082e4c4b70d7d60d9747230426c0f299c63e73f410"
+  "signature": "sha256:88299b8bf8496d8642bbdee38eb30e47846ebd319370af9c11d8e7f7da1a4542"
  },
  "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,