You can now specify an optional custom message in .feed and .yun which will be posted along with an update

src/NadekoBot/Db/Models/FeedSub.cs

@@ -8,6 +8,8 @@ public class FeedSub : DbEntity
 
     public ulong ChannelId { get; set; }
     public string Url { get; set; }
+    
+    public string Message { get; set; }
 
     public override int GetHashCode()
         => Url.GetHashCode(StringComparison.InvariantCulture) ^ GuildConfigId.GetHashCode();