Add example of HelpProvider + README

Relocate detailed help docs to main README

_examples/shell/help/README.md

@@ -0,0 +1,3 @@
+# Example of custom help providers
+
+This example demonstrates how to add `Help() string` functions (ie. the `HelpProvider` interface) to your commands, arguments, and flags to augment the help text provided using `help:""` style tagged annotations.

_examples/shell/help/main.go

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+package main
+
+import (
+	"fmt"
+
+	"github.com/alecthomas/kong"
+)
+
+var cli struct {
+	Flag flagWithHelp    `help:"Regular flag help"`
+	Echo commandWithHelp `cmd:"" help:"Regular command help"`
+}
+
+type flagWithHelp bool
+
+func (f *flagWithHelp) Help() string {
+	return "🏁 additional flag help"
+}
+
+type commandWithHelp struct {
+	Msg argumentWithHelp `arg:"" help:"Regular argument help"`
+}
+
+func (c *commandWithHelp) Help() string {
+	return "🚀 additional command help"
+}
+
+type argumentWithHelp struct {
+	Msg string `arg:""`
+}
+
+func (f *argumentWithHelp) Help() string {
+	return "📣 additional argument help"
+}
+
+func main() {
+	ctx := kong.Parse(&cli,
+		kong.Name("help"),
+		kong.Description("An app demonstrating HelpProviders"),
+		kong.UsageOnError(),
+		kong.ConfigureHelp(kong.HelpOptions{
+			Compact: true,
+			Summary: false,
+		}))
+	switch ctx.Command() {
+	case "echo <msg>":
+		fmt.Println(cli.Echo.Msg)
+	}
+}