README: major rewrite, with embedded example

Reconstruct README to better promote the project.

README.md

@@ -5,11 +5,39 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/dolmen-go/jsonptr)](https://goreportcard.com/report/github.com/dolmen-go/codegen)
 
 
-Utilities for writing code generators from a [text template](https://golang.org/pkg/text/template/),
-with output properly formatted thanks to [`gofmt`](https://golang.org/pkg/go/format/).
+Package `codegen` provides a Go API for writing Go code generators from a [`text/template`](https://pkg.go.dev/text/template).
+
+```go
+const tmpl = `// Code generated by main.go; DO NOT EDIT.
+
+package main
+
+const x = {{ printf "%q" .str }}
+
+var v = {{ printf "%#v" .array }}
+`
+
+func main() {
+	codegen.MustParse(tmpl).CreateFile(
+		"main_gen.go",
+		map[string]any{
+			"str":   `abcdef`,
+			"array": []int{1, 2, 3},
+		},
+	)
+}
+```
+
+[Try on the Go Playground](https://go.dev/play/p/CfJBt9ng-4w).
+
+Features:
+* The Go syntax is validated before saving.
+* The output properly is reformatted thanks to [`gofmt`](https://pkg.go.dev/go/format).
+* The Go standard for marking generated code (https://golang.org/s/generatedcode) is enforces.
 
 See [documentation](https://godoc.org/github.com/dolmen-go/codegen) and [example](example_test.go).
 
+
 ## License
 
 Copyright 2016-2024 Olivier Mengué