Expanding CLI docs

Adding new pages for commandApp, commands and settings.

docs/input/cli/commands.md

@@ -0,0 +1,63 @@
+Title: Creating Commands
+Order: 6
+---
+
+Commands in `Spectre.Console.Cli` are defined by creating a class that inherits from either `Spectre.Console.Cli.Command<TSettings>` or `Spectre.Console.Cli.AsyncCommand<TSettings>`. `Command<TSettings>` must implement an `Execute` method that returns an int where as `AsyncCommand<TSettings>` must implement `ExecuteAsync`  returning `Task<int>`.
+
+```csharp
+public class HelloCommand : Command<HelloCommand.Settings>
+{
+    public class Settings : LogCommandSettings
+    {
+        [CommandArgument(0, "[Name]")]
+        public string Name { get; set; }
+    }
+
+
+    public override int Execute(CommandContext context, Settings settings)
+    {
+        AnsiConsole.MarkupLine($"Hello, [blue]{settings.Name}[/]");
+        return 0;
+    }
+}
+```
+
+# Configuring.
+
+Commands are configured via the [`CommandApp`](commandApp)'s `Configure` method.
+
+```csharp
+var app = new CommandApp();
+app.Configure(config =>
+{
+    config.AddCommand<HelloCommand>("hello")
+        .WithAlias("hola")
+        .WithDescription("Say hello")
+        .WithExample(new []{"hello", "Phil"})
+        .WithExample(new []{"hello", "Phil", "--count", "4"});
+});
+```
+
+* `WithAlias` allows commands to have multiple names.
+* `WithDescription` is used by the help renderer to give commands a description when displaying help.
+* `WithExample` is used by the help renderer to provide examples to the user for running the commands. The parameters is a string array that matches the values passed in `Main(string[] args)`.
+
+# Dependency Injection.
+
+Constructor injection is supported on commands. See the [`CommandApp`](commandApp) documentation for further information on configuring `Spectre.Console` for your container.
+
+# Validation.
+
+While the settings can validate themselves, the command also provides a validation. For example, `IFileSystem` might be injected into the command which we want to use to validate that a path passed in exists.
+
+```csharp
+public override ValidationResult Validate(CommandContext context, Settings settings)
+{
+    if (_fileSystem.IO.File.Exists(settings.Path))
+    {
+        return ValidationResult.Error($"Path not found - {settings.Path}");
+    }
+
+    return base.Validate(context, settings);
+}
+```