Make AsyncCommand a little more discoverable (#375)

docs/input/cli/introduction.md

@@ -75,6 +75,8 @@ public class AddReferenceCommand : Command<AddReferenceSettings>
 }
 ```
 
+You can use `AsyncCommand` if you need async support.
+
 ## Let's tie it together
 
 Now when we have our commands and settings implemented, we can compose a command tree

src/Spectre.Console/Cli/Command.cs

@@ -5,6 +5,7 @@ namespace Spectre.Console.Cli
     /// <summary>
     /// Base class for a command without settings.
     /// </summary>
+    /// <seealso cref="AsyncCommand"/>
     public abstract class Command : ICommand<EmptyCommandSettings>
     {
         /// <summary>

src/Spectre.Console/Cli/Command`1.cs

@@ -8,6 +8,7 @@ namespace Spectre.Console.Cli
     /// Base class for a command.
     /// </summary>
     /// <typeparam name="TSettings">The settings type.</typeparam>
+    /// <seealso cref="AsyncCommand{TSettings}"/>
     public abstract class Command<TSettings> : ICommand<TSettings>
         where TSettings : CommandSettings
     {