added asset role description field

com.unity.perception/Runtime/Randomization/Randomizers/AssetSources/AssetRoleBase.cs

         /// The string label uniquely associated with this asset role
         /// </summary>
         public abstract string label { get; }
+
+        /// <summary>
+        /// A description for this asset role
+        /// </summary>
+        public abstract string description { get; }
     }
 }