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/03_sharded_client.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <RootNamespace>_03_sharded_client</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Discord.Net.Commands\Discord.Net.Commands.csproj" />
+    <ProjectReference Include="..\..\src\Discord.Net.WebSocket\Discord.Net.WebSocket.csproj" />
+  </ItemGroup>
+
+</Project>

samples/03_sharded_client/Modules/PublicModule.cs

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+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);
+        }
+    }
+}

samples/03_sharded_client/Program.cs

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+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;
+        }
+    }
+}

samples/03_sharded_client/Services/CommandHandlingService.cs

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+using System;
+using System.Reflection;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using Discord;
+using Discord.Commands;
+using Discord.WebSocket;
+
+namespace _03_sharded_client.Services
+{
+    public class CommandHandlingService
+    {
+        private readonly CommandService _commands;
+        private readonly DiscordShardedClient _discord;
+        private readonly IServiceProvider _services;
+
+        public CommandHandlingService(IServiceProvider services)
+        {
+            _commands = services.GetRequiredService<CommandService>();
+            _discord = services.GetRequiredService<DiscordShardedClient>();
+            _services = services;
+        }
+
+        public async Task InitializeAsync()
+        {
+            await _commands.AddModulesAsync(Assembly.GetEntryAssembly(), _services);
+            _discord.MessageReceived += MessageReceivedAsync;
+        }
+
+        public async Task MessageReceivedAsync(SocketMessage rawMessage)
+        {
+            // Ignore system messages, or messages from other bots
+            if (!(rawMessage is SocketUserMessage message))
+                return;
+            if (message.Source != MessageSource.User)
+                return;
+
+            // This value holds the offset where the prefix ends
+            var argPos = 0;
+            if (!message.HasMentionPrefix(_discord.CurrentUser, ref argPos))
+                return;
+
+            // A new kind of command context, ShardedCommandContext can be utilized with the commands framework
+            var context = new ShardedCommandContext(_discord, message);
+            var result = await _commands.ExecuteAsync(context, argPos, _services);
+
+            if (result.Error.HasValue &&
+                result.Error.Value != CommandError.UnknownCommand) // it's bad practice to send 'unknown command' errors
+                await context.Channel.SendMessageAsync(result.ToString());
+        }
+    }
+}