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/Program.cs

@@ -0,0 +1,69 @@
+using System;
+using System.Threading.Tasks;
+using _03_sharded_client.Services;
+using Discord;
+using Discord.Commands;
+using Discord.WebSocket;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace _03_sharded_client
+{
+    // This is a minimal example of using Discord.Net's Sharded Client
+    // The provided DiscordShardedClient class simplifies having multiple
+    // DiscordSocketClient instances (or shards) to serve a large number of guilds.
+    class Program
+    {
+        private DiscordShardedClient _client;
+
+        static void Main(string[] args)
+            => new Program().MainAsync().GetAwaiter().GetResult();
+        public async Task MainAsync()
+        {
+            // You specify the amount of shards you'd like to have with the
+            // DiscordSocketConfig. Generally, it's recommended to 
+            // have 1 shard per 1500-2000 guilds your bot is in.
+            var config = new DiscordSocketConfig
+            {
+                TotalShards = 2
+            };
+
+            _client = new DiscordShardedClient(config);
+            var services = ConfigureServices();
+
+            // The Sharded Client does not have a Ready event.
+            // The ShardReady event is used instead, allowing for individual
+            // control per shard.
+            _client.ShardReady += ReadyAsync;
+            _client.Log += LogAsync;
+
+            await services.GetRequiredService<CommandHandlingService>().InitializeAsync();
+
+            await _client.LoginAsync(TokenType.Bot, Environment.GetEnvironmentVariable("token"));
+            await _client.StartAsync();
+
+            await Task.Delay(-1);
+        }
+
+        private IServiceProvider ConfigureServices()
+        {
+            return new ServiceCollection()
+                .AddSingleton(_client)
+                .AddSingleton<CommandService>()
+                .AddSingleton<CommandHandlingService>()
+                .BuildServiceProvider();
+        }
+
+
+        private Task ReadyAsync(DiscordSocketClient shard)
+        {
+            Console.WriteLine($"Shard Number {shard.ShardId} is connected and ready!");
+            return Task.CompletedTask;
+        }
+
+        private Task LogAsync(LogMessage log)
+        {
+            Console.WriteLine(log.ToString());
+            return Task.CompletedTask;
+        }
+    }
+}