Flesh out README with an example

Author: ammario

README.md

@@ -1,4 +1,72 @@
 # serpent
 
-`serpent` is a CLI configuration framework based on [cobra](https://github.com/spf13/cobra).
+[![Go Reference](https://pkg.go.dev/badge/github.com/coder/serpent.svg)](https://pkg.go.dev/github.com/coder/serpent)
 
+`serpent` is a CLI configuration framework based on [cobra](https://github.com/spf13/cobra) and used by [coder/coder](https://github.com/coder/coder).
+It's designed for large-scale CLIs with dozens of commands and hundreds
+of options. If you're building a small, self-contained tool, go with
+cobra.
+
+## Basic Usage
+
+See `example/echo`:
+
+```go
+package main
+
+import (
+	"os"
+	"strings"
+
+	"github.com/coder/serpent"
+)
+
+func main() {
+	var upper bool
+	cmd := serpent.Cmd{
+		Use:   "echo <text>",
+		Short: "Prints the given text to the console.",
+		Options: serpent.OptionSet{
+			{
+				Name:        "upper",
+				Value:       serpent.BoolOf(&upper),
+				Flag:        "upper",
+				Description: "Prints the text in upper case.",
+			},
+		},
+		Handler: func(inv *serpent.Invocation) error {
+			if len(inv.Args) == 0 {
+				inv.Stderr.Write([]byte("error: missing text\n"))
+				os.Exit(1)
+			}
+
+			text := inv.Args[0]
+			if upper {
+				text = strings.ToUpper(text)
+			}
+
+			inv.Stdout.Write([]byte(text))
+			return nil
+		},
+	}
+
+	err := cmd.Invoke().WithOS().Run()
+	if err != nil {
+		panic(err)
+	}
+}
+```
+
+## Design
+This Design section assumes you have a good understanding of how `cobra` works.
+
+### Options
+
+Serpent is designed for high-configurability. To us, that means providing
+many ways to configure the same value (env, YAML, flags, etc.) and keeping
+the code clean and testable as you scale the number of options.
+
+
+## More coming...
+This README is a stub for now. We'll better explain the design and usage
+of `serpent` in the future.

cmd.go

@@ -76,10 +76,10 @@ func (c *Cmd) Walk(fn func(*Cmd)) {
 	}
 }
 
-// PrepareAll performs initialization and linting on the command and all its children.
-func (c *Cmd) PrepareAll() error {
+// init performs initialization and linting on the command and all its children.
+func (c *Cmd) init() error {
 	if c.Use == "" {
-		return xerrors.New("command must have a Use field so that it has a name")
+		c.Use = "unnamed"
 	}
 	var merr error
 
@@ -116,7 +116,7 @@ func (c *Cmd) PrepareAll() error {
 	})
 	for _, child := range c.Children {
 		child.Parent = c
-		err := child.PrepareAll()
+		err := child.init()
 		if err != nil {
 			merr = errors.Join(merr, xerrors.Errorf("command %v: %w", child.Name(), err))
 		}
@@ -493,6 +493,11 @@ func findArg(want string, args []string, fs *pflag.FlagSet) (int, error) {
 //
 //nolint:revive
 func (inv *Invocation) Run() (err error) {
+	err = inv.Command.init()
+	if err != nil {
+		return xerrors.Errorf("initializing command: %w", err)
+	}
+
 	defer func() {
 		// Pflag is panicky, so additional context is helpful in tests.
 		if flag.Lookup("test.v") == nil {

example/echo/main.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"os"
+	"strings"
+
+	"github.com/coder/serpent"
+)
+
+func main() {
+	var upper bool
+	cmd := serpent.Cmd{
+		Use:   "echo <text>",
+		Short: "Prints the given text to the console.",
+		Options: serpent.OptionSet{
+			{
+				Name:        "upper",
+				Value:       serpent.BoolOf(&upper),
+				Flag:        "upper",
+				Description: "Prints the text in upper case.",
+			},
+		},
+		Handler: func(inv *serpent.Invocation) error {
+			if len(inv.Args) == 0 {
+				inv.Stderr.Write([]byte("error: missing text\n"))
+				os.Exit(1)
+			}
+
+			text := inv.Args[0]
+			if upper {
+				text = strings.ToUpper(text)
+			}
+
+			inv.Stdout.Write([]byte(text))
+			return nil
+		},
+	}
+
+	err := cmd.Invoke().WithOS().Run()
+	if err != nil {
+		panic(err)
+	}
+}