Add a little bit more information to the way DTC material works

Thanks @rmsy for clearing this up!

Author: Harry

src/modules/gamemode_dtc.haml

@@ -10,6 +10,7 @@
                     %h1 Destroy the Core
                 :markdown
                     Players have to locate and break the enemy teams core, usually an obsidian sphere filled with lava. The lava has to leak out a certain amount for the game to end. Teams can have more than one core. Cores automatically change from their base material to gold after 15 minutes and glass after 20.
+                    The materials attribute follows the system of Obsidian -> Gold, Gold -> Glass system. However if you set the material to anything other than Obsidian, Gold or Glass, it will not cycle to another block type.
 
                         <cores material="obsidian" leak="8">
                             <core team="blue"><cuboid min="10,15,12" max="12,13,16"/></core>