Add docs for the Command Service

docs/guides/samples/dependency_map_setup.cs

@@ -0,0 +1,16 @@
+using Discord;
+using Discord.Commands;
+
+public class Commands
+{
+    public async Task Install(DiscordSocketClient client)
+    {
+        var commands = new CommandService();
+        var map = new DependencyMap();
+        map.Add<IDiscordClient>(client);
+        var self = await client.GetCurrentUserAsync();
+        map.Add<ISelfUser>(self);
+        await commands.LoadAssembly(Assembly.GetCurrentAssembly(), map);
+    }
+    // ...
+}