docs: mention new promslog package in package list in README

Signed-off-by: TJ Hoplock <t.hoplock@gmail.com>

README.md

@@ -10,6 +10,7 @@ any stability guarantees for external usage.
 * **expfmt**: Decoding and encoding for the exposition format
 * **model**: Shared data structures
 * **promlog**: A logging wrapper around [go-kit/log](https://github.com/go-kit/kit/tree/master/log)
+* **promslog**: A logging wrapper around [log/slog](https://pkg.go.dev/log/slog)
 * **route**: A routing wrapper around [httprouter](https://github.com/julienschmidt/httprouter) using `context.Context`
 * **server**: Common servers
 * **version**: Version information and metrics