Add table of contents

docs/assertions.md

@@ -1,6 +1,12 @@
 <a id="top"></a>
 # Assertion Macros
 
+**Contents**  
+[Natural Expressions](#natural-expressions)  
+[Exceptions](#exceptions)  
+[Matcher expressions](#matcher-expressions)  
+[Thread Safety](#thread-safety)  
+
 Most test frameworks have a large collection of assertion macros to capture all possible conditional forms (```_EQUALS```, ```_NOTEQUALS```, ```_GREATER_THAN``` etc).
 
 Catch is different. Because it decomposes natural C-style conditional expressions most of these forms are reduced to one or two that you will use all the time. That said there are a rich set of auxilliary macros as well. We'll describe all of these here.