[Bf-docboard] Minor updates in the Material chapter

[Olivier SARAJA (Linuxgraphic)]

Hi,

because I am short on free time, I only did the few following minor changes:
* reworked the introductory page, and included a few words about how 
raytracing influences shading
* added the Minnaert shader in the Diffuse shaders list
* added the Ward Isotropic shader in the Specular shaders list

Please note I used Jorge's explanations found here as a basis: 
http://worldspace.berlios.de/openal/temporal/Wardisotropic.html

I'm waiting for his permission, so if I don't get it, I'll have to rework this 
part a little more.

If you want to edit/correct grammar/spelling/ideas, please feel free to do so 
(I removed the WIP tags on these few pages).

The next main step will be to reorganize the ''Materials in practice'' part. I 
will certainly break it in several smaller parts, and try to mirror actual 
shading pannels as much as possible.

Sorry for not being able to work more heavily on the docs for now, but free 
time is really scarce for me these days.

Merry Christmas to all! :)))

-- 
Olivier SARAJA