add usage and credits sections to readme

Author: hermannm

README.md

@@ -1,5 +1,51 @@
 # devlog
 
-A structured logging handler for Go, with a human-readable output format designed for development builds.
+A structured logging handler for Go, with a human-readable output format designed for development
+builds.
 
 Run `go get hermannm.dev/devlog` to add it to your project!
+
+## Usage
+
+`devlog.Handler` implements [`slog.Handler`](https://pkg.go.dev/log/slog#Handler), so it can handle
+output for `slog`'s logging functions. It can be configured as follows:
+
+```go
+logger := slog.New(devlog.NewHandler(os.Stdout, nil))
+slog.SetDefault(logger)
+```
+
+Logging with `slog` will now use this handler:
+
+```go
+slog.Warn("No value found for 'PORT' in env, defaulting to 8000")
+slog.Info("Server started", slog.Int("port", 8000), slog.String("environment", "DEV"))
+slog.Error(
+	"Database query failed",
+	slog.Group("dbError", slog.Int("code", 60), slog.String("message", "UNKNOWN_TABLE")),
+)
+```
+
+...giving the following output (using a gruvbox terminal color scheme):
+
+![Screenshot of 3 log messages in a terminal](https://github.com/hermannm/devlog/blob/assets/devlog-example-output.png?raw=true)
+
+This output is meant to be easily read by a developer working locally. However, you may want a more
+structured format for production, such as JSON, to make log analysis easier. You can get both by
+conditionally choosing the log handler for your application, e.g.:
+
+```go
+var handler slog.Handler
+switch os.Getenv("ENVIRONMENT") {
+case "PROD":
+	handler = slog.NewJSONHandler(os.Stdout, nil)
+case "DEV":
+	handler = devlog.NewHandler(os.Stdout, nil)
+}
+
+slog.SetDefault(slog.New(handler))
+```
+
+## Credits
+
+- [Jonathan Amsterdam](https://github.com/jba) for his fantastic [Guide to Writing `slog` Handlers](https://github.com/golang/example/blob/1d6d2400d4027025cb8edc86a139c9c581d672f7/slog-handler-guide/README.md)