add TOC with link to API docs

Author: floe

README.md

@@ -1,5 +1,14 @@
 # libfreenect2
 
+## Table of Contents
+
+* [API Documentation (external)](https://openkinect.github.io/libfreenect2/)
+* [Description](README.md#Description)
+* [Requirements](README.md#Requirements)
+* [FAQ](README.md#FAQ)
+* [Maintainers](README.md#Maintainers)
+* [Installation](README.md#Installation)
+
 ## Description
 Driver for Kinect for Windows v2 (K4W2) devices (release and developer preview).