docs: add example to README (#3)

Co-authored-by: Eng Zer Jun <[email protected]>

Author: 6543

README.md

@@ -4,5 +4,81 @@
 [![Go Reference](https://pkg.go.dev/badge/github.com/urfave/cli-docs/v3.svg)](https://pkg.go.dev/github.com/urfave/cli-docs/v3)
 [![Go Report Card](https://goreportcard.com/badge/github.com/urfave/cli-docs/v3)](https://goreportcard.com/report/github.com/urfave/cli-docs/v3)
 
-urfave/cli-docs/v3 is an extended documentation library for use
-with urfave/cli/v3.
+urfave/cli-docs/v3 is an extended documentation library for use with urfave/cli/v3.
+
+## Start using
+
+1. Add the dependency to your project
+
+```sh
+ go get github.com/urfave/cli-docs/v3@latest
+ ```
+
+2. Add it as import
+
+```diff
+ import (
++  docs "github.com/urfave/cli-docs/v3"
+ )
+```
+
+3. Now use it e.g. to generate markdown document from a command
+
+```go
+package main
+
+import (
+    "context"
+    "fmt"
+    "os"
+
+    docs "github.com/urfave/cli-docs/v3"
+    cli "github.com/urfave/cli/v3"
+)
+
+func main() {
+    app := &cli.Command{
+        Name:  "greet",
+        Usage: "say a greeting",
+        Action: func(ctx context.Context, c *cli.Command) error {
+            fmt.Println("Greetings")
+            return nil
+        },
+    }
+
+    md, err := docs.ToMarkdown(app)
+    if err != nil {
+        panic(err)
+    }
+
+    fi, err := os.Create("cli-docs.md")
+    if err != nil {
+        panic(err)
+    }
+    defer fi.Close()
+    if _, err := fi.WriteString("# CLI\n\n" + md); err != nil {
+        panic(err)
+    }
+}
+```
+
+This will create a file `cli-docs.md` with content:
+
+````md
+# CLI
+
+# NAME
+
+greet - say a greeting
+
+# SYNOPSIS
+
+greet
+
+**Usage**:
+
+```
+greet [GLOBAL OPTIONS] [command [COMMAND OPTIONS]] [ARGUMENTS...]
+```
+
+````