Sectionalize API docs better and update README ToC

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

API.md

@@ -1,7 +1,8 @@
 API Documentation
------------------
+=================
 
-* [API](#api)
+* [Introduction](#introduction)
+* [Overview](#overview)
   * [Create an Event Context](#create-an-event-context)
   * [Register an Event Watcher](#register-an-event-watcher)
   * [Start Event Loop](#start-event-loop)
@@ -10,6 +11,9 @@ API Documentation
 * [Joystick Example](#joystick-example)
 
 
+Introduction
+------------
+
 The C API to [libuEv][], listed in `uev/uev.h`, handles three different
 types of events: I/O (files, sockets, message queues, etc.), timers, and
 signals.  The [Summary](#summary) details a slight caveat on signals.
@@ -25,6 +29,10 @@ the wall clock changes, either via NTP or user input.
   other C libraries have no known fix.  It is strongly recommended to
   use relative timers as often as possible.
 
+
+Overview
+--------
+
 ```C
 /*
  * Callback example, arg comes from the watcher's *_init() function,

README.md

@@ -4,7 +4,7 @@ libuEv | Simple event loop for Linux
 
 
 * [Introduction](#introduction)
-* [API](API.md#api)
+* [API](API.md#overview)
   * [Create an Event Context](API.md#create-an-event-context)
   * [Register an Event Watcher](API.md#register-an-event-watcher)
   * [Start Event Loop](API.md#start-event-loop)