Add json properties to difficulty attributes

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyAttributes.cs

@@ -1,12 +1,14 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using Newtonsoft.Json;
 using osu.Game.Rulesets.Difficulty;
 
 namespace osu.Game.Rulesets.Catch.Difficulty
 {
     public class CatchDifficultyAttributes : DifficultyAttributes
     {
+        [JsonProperty("approach_rate")]
         public double ApproachRate { get; set; }
     }
 }