[Gtkradiant] Mission statement - declaring current build official

[Timothee Besset]

I've updated the landing page for the site with a short mission
statement: http://icculus.org/gtkradiant/

Please let me know if you have comments. This should be in line with
the various discussions we've had on IRC about where I feel we should
be with this project.

Also, I would like to make the current build official. It's quite
stable, no big blocking issues. On the next cycle we can make it
official and have some sort of release party.

Thanks to everyone who has been giving feedback, sending issues and patches.

TTimo