Made all traitInfo fields readonly

This came up while working on the new documentation generation and comparing the results to ORAIDE's own code parser.

OpenRA.Mods.Common/Traits/Conditions/SpreadsCondition.cs

@@ -28,7 +28,7 @@ namespace OpenRA.Mods.Common.Traits
 		public readonly string SpreadCondition = "spreading";
 
 		[Desc("Time in ticks to wait between spreading further.")]
-		public int Delay = 5;
+		public readonly int Delay = 5;
 
 		public override object Create(ActorInitializer init) { return new SpreadsCondition(this); }
 	}