Add external references to the documentation

[robo9k]

The documentation should mention that Quake 3 itself is poorly documented and link to helpful ressources such as:

• http://fabiensanglard.net/quake3/
• https://www.icculus.org/~phaethon/q3mc/skel.html
• http://www.soclose.de/q3doc/
• http://wiki.ioquake3.org/
• https://github.com/robo9k/ioq3-doc (shameless plug, not quite helpful yet 😄 )
• ...