document SAO algorithm (close #389)

h3d/shader/SAO.hx

@@ -1,5 +1,9 @@
 package h3d.shader;
 
+/**
+	Screen space ambient occlusion.
+	Uses "Scalable Ambient Obscurance" [McGuire12]
+**/
 class SAO extends ScreenShader {
 
 	static var SRC = {