Add authors comment that suggest public-domain license.

Sourceforge was dead for more than a week therefore add the license
information. I could not find the original TGM source (dead link) so I'm not
even sure if this still applies or if the glsl was totally rewritten. None
of the glsl files have a copyright header so it's hard to tell.

plugins/GSdx/res/glsl/shadeboost.glsl

@@ -4,6 +4,9 @@
 ** Contrast, saturation, brightness
 ** Code of this function is from TGM's shader pack
 ** http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=21057
+** TGM's author comment about the license (included in the previous link)
+** "do with it, what you want! its total free!
+** (but would be nice, if you say that you used my shaders  :wink: ) but not necessary"
 */
 
 struct vertex_basic