update docs

Author: arianvp

README.md

@@ -6,12 +6,56 @@
 > Journald only supports keys of the form `^[A-Z_][A-Z0-9_]*$`. Any other keys will be silently dropped.
 
 ```go
-h := slogjournal.NewHandler(nil)
+h , err := slogjournal.NewHandler(nil)
 log := slog.New(h)
 log.Info("Hello, world!", "EXTRA_KEY", "5")
 log.Info("Hello, world!", slog.Group("HTTP", "METHOD", "put", "URL", "http://example.com"))
 ```
 
 
 > [!CAUTION]
-> This is pre-release software. No releases have been tagged yet.
+> This is pre-release software. No releases have been tagged yet.
+
+
+### Make sure your logs are compatible with the journal
+
+When using third-party slog libraries, you do not have control over the attributes that are passed to the logger.
+Because the journal only supports keys of the form `^[A-Z_][A-Z0-9_]*$`, you may need to transform keys that don't match this pattern.
+For this you can use the `ReplaceGroup` and `ReplaceAttr` fields in `Options`:
+
+
+```go
+import (
+    "log/slog"
+    sloghttp "github.com/samber/slog-http"
+    slogjournal "github.com/systemd/slog-journal"
+)
+
+h , err := slogjournal.NewHandler(&slogjournal.Options{
+    ReplaceGroup: func(k string) string {
+        return strings.ReplaceAll(strings.ToUpper(k), "-", "_")
+    },
+    ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
+        a.Key = strings.ReplaceAll(strings.ToUpper(a.Key), "-", "_")
+        return a
+    },
+})
+
+log := slog.New(h)
+mux := http.NewServeMux()
+mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+    log.Info("Hello world")
+    w.Write([]byte("Hello, world!"))
+})
+http.ListenAndServe(":8080", sloghttp.New(log)(mux))
+```
+
+
+
+
+
+
+
+
+
+```

journal.go

@@ -1,3 +1,10 @@
+/*
+Package slogjournal provides a handler for the systemd journal.
+
+The journal only accepts keys of the form ^[A-Z_][A-Z0-9_]*$.
+
+Example:
+*/
 package slogjournal
 
 import (