docs: add project description (#5)

Author: tmzane

README.md

@@ -5,4 +5,39 @@
 [![goreportcard](https://goreportcard.com/badge/go-simpler.org/slog-gen)](https://goreportcard.com/report/go-simpler.org/slog-gen)
 [![codecov](https://codecov.io/gh/go-simpler/slog-gen/branch/main/graph/badge.svg)](https://codecov.io/gh/go-simpler/slog-gen)
 
-Generate domain-specific `slog.Attr` constructors.
+## 📌 About
+
+When using `log/slog` in a production-grade project, it is useful to write helpers to avoid human error in the keys.
+
+```go
+slog.Info("a user has logged in", "user_id", 42)
+slog.Info("a user has logged out", "user_ip", 42) // oops :(
+```
+
+Depending on your code style, these can be simple constants (if you prefer key-value arguments)...
+
+```go
+const UserId = "user_id"
+```
+
+...or constructors for `slog.Attr` (if you're a safety/performance advocate).
+
+```go
+func UserId(value int) slog.Attr {
+    return slog.Int("user_id", value)
+}
+```
+
+`slog-gen` generates such code for you based on a simple config (a single source of truth),
+which makes it easy to share domain-specific helpers between related (micro)services.
+
+## 📦 Install
+
+Create and fill in the `.slog.yml` config based on the example,
+then add the following directive to any `.go` file and run `go generate ./...`.
+
+```go
+//go:generate go run go-simpler.org/slog-gen --config=.slog.yml
+```
+
+To get started, see the `.slog.example.yml` file and the `example` directory.

codegen_test.go

@@ -9,7 +9,7 @@ import (
 	. "go-simpler.org/assert/dotimport"
 )
 
-//go:generate go run main.go codegen.go -config=.slog.example.yml
+//go:generate go run main.go codegen.go --config=.slog.example.yml
 
 func Test_readConfig(t *testing.T) {
 	r := strings.NewReader(`