Update README.md

Added a TOC.

README.md

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 Air to air combat sandbox, created in Python 3 using the [HARFANG 3D 2 framework](https://www.harfang3d.com).
 
+**Table of content**
+
+- [Overview](#dogfight-2-air-to-air-combat-sandbox)
+- [How to run the Dogfight Sandbox](#how-to-run-the-dogfight-sandbox)
+- [About VR mode](#about-vr-mode)
+- [Network mode overview](#network-mode-overview)
+- [Aircraft Commands](#aircraft-keyboard-commands)
+- [Contributors](#contributors)
+- [Screenshots](#screenshots)
+- [Publications and citations](#publications-and-citations)
+
 [![](screenshots/dogfight-sandbox-python-vr-yt.png)](https://www.youtube.com/watch?v=4Q2f0dKh8vE)
 
 The game features :