Add DiscordShardedClient sample project & Client FAQ entry. (#1177)

* Add DiscordShardedClient sample project & Client FAQ entry.

* Revise language, fix typo, add xrefs

* Adjust placement of message handler.

* Resolve DI issue with initialized client; properly initialize command handling service.

samples/03_sharded_client/Modules/PublicModule.cs

@@ -0,0 +1,17 @@
+using System.Threading.Tasks;
+using Discord.Commands;
+
+namespace _03_sharded_client.Modules
+{
+    // Remember to make your module reference the ShardedCommandContext
+    public class PublicModule : ModuleBase<ShardedCommandContext>
+    {
+        [Command("info")]
+        public async Task InfoAsync()
+        {
+            var msg = $@"Hi {Context.User}! There are currently {Context.Client.Shards} shards!
+                This guild is being served by shard number {Context.Client.GetShardFor(Context.Guild).ShardId}";
+            await ReplyAsync(msg);
+        }
+    }
+}