README.md: Add table of contents

Author: roberthoenig

README.md

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+Table of Contents
+=================
+
+   * [FirstOrderLogic.jl](#firstorderlogicjl)
+      * [Overview](#overview)
+         * [Functionality](#functionality)
+         * [Purpose](#purpose)
+      * [Installation](#installation)
+      * [Usage](#usage)
+         * [Quick walkthrough](#quick-walkthrough)
+         * [Formula syntax](#formula-syntax)
+            * [Example](#example)
+         * [Some important functions](#some-important-functions)
+      * [How is this different from PROLOG?](#how-is-this-different-from-prolog)
+
 # FirstOrderLogic.jl
 
 This is a Julia package for parsing, manipulating and evaluating formulas in first-order logic.