added NewSlogLoggerWithLevelMapping (#510)

* added NewSlogLoggerWithLevelMapping

* added docs

* gitignore hugo stuff

.gitignore

@@ -5,6 +5,7 @@ docs/node_modules/
 docs/public/
 docs/content/src-link
 docs/content/middleware
+docs/hugo_stats.json
 *.out
 *.log
 .mod-cache

docs/content/docs/getting-started.md

@@ -348,6 +348,9 @@ The complete example's source can be found at [/_examples/basic/3-router/main.go
 To see Watermill's logs, pass any logger that implements the [LoggerAdapter](https://github.com/ThreeDotsLabs/watermill/blob/master/log.go).
 For experimental development, you can use `NewStdLogger`.
 
+Watermill provides ready-to-use `slog` adapter. You can create it with [`watermill.NewSlogLogger`](https://github.com/ThreeDotsLabs/watermill/blob/master/slog.go).
+You can also map Watermill's log levels to `slog` levels with [`watermill.NewSlogLoggerWithLevelMapping`](https://github.com/ThreeDotsLabs/watermill/blob/master/slog.go).
+
 ## What's next?
 
 For more details, see [documentation topics]({{< ref "/docs" >}}).