add materials (mat) files and readme.md

cstrike/models/p_usp.mdl/p_usp.mdl.mat

@@ -0,0 +1,8 @@
+
+{
+"for" "#models/p_usp/usp_skin.mdl"
+"basecolor_map" "usp_skin.png"
+//"normal_map" "usp_skin_normal.png"
+//"metal_map" "usp_skin_metal.png"
+//"roughness_map" "usp_skin_roughness.png"
+}