Describe machine upgrades in guide

guide.lua

@@ -1184,6 +1184,28 @@ local pages={
 				<left>||Picture 1. Wind Mill producing some power.||</left>
 			]]
 		}
+	},
+
+	{
+		name="upgrades",
+		title=S("Machine upgrades"),
+		icon="industrialtest:overclocker_upgrade",
+		content={
+			[[
+				<big>||Machine upgrades||</big>
+				<left>
+					||Upgrades can be put into special slots in machine formspec for this purpose. They increase machine capabilities while changing other properties. Not all machines can accept upgrades, however this can be determined by presence of four slots on right of formspec where upgrades are put. The following are all machine upgrades:||
+				</left>
+				<left>
+					||<item name="industrialtest:overclocker_upgrade" height="{{ITEM_HEIGHT}}"> <b>Overclocker Upgrade</b> - speeds up the machine.||
+					||<item name="industrialtest:power_storage_upgrade" height="{{ITEM_HEIGHT}}"> <b>Power Storage Upgrade</b> - increases the amount of power that machine can store.||
+					||<item name="industrialtest:transformer_upgrade" height="{{ITEM_HEIGHT}}"> <b>Transformer Upgrade</b> - allows machine to be connected to higher voltage by one level.||
+				</left>
+				<mono>--</mono>
+				<img name="industrialtest_guide_upgrades.png">
+				<left>||Picture 1. <item name="industrialtest:electric_furnace" height="{{ITEM_HEIGHT}}"> <b>Electric Furnace</b> with <item name="industrialtest:overclocker_upgrade" height="{{ITEM_HEIGHT}}"> <b>Overclocker Upgrade</b>.||</left>
+			]]
+		}
 	}
 }