add a None flag to SystemChannelMessage enum

src/Discord.Net.Core/Entities/Guilds/SystemChannelMessageDeny.cs

@@ -5,6 +5,11 @@ namespace Discord
     [Flags]
     public enum SystemChannelMessageDeny
     {
+        /// <summary>
+        ///     Deny none of the system channel messages.
+        ///     This will enable all of the system channel messages.
+        /// </summary>
+        None = 0,
         /// <summary>
         ///     The messages that are sent when a user joins the guild.
         /// </summary>