Add a note that puma-plugin-systemd is redundant now

By now Puma has gained all systemd integrations that puma-plugin-systemd
used to provide. While I didn't check, there's a good chance the plugin
now conflicts with the integrated features conflict. Users should prefer
native support.

README.md

@@ -375,7 +375,7 @@ Community guides:
 
 * [puma-metrics](https://github.com/harmjanblok/puma-metrics) — export Puma metrics to Prometheus
 * [puma-plugin-statsd](https://github.com/yob/puma-plugin-statsd) — send Puma metrics to statsd
-* [puma-plugin-systemd](https://github.com/sj26/puma-plugin-systemd) — deeper integration with systemd for notify, status and watchdog
+* [puma-plugin-systemd](https://github.com/sj26/puma-plugin-systemd) — deeper integration with systemd for notify, status and watchdog; now all integrated in Puma itself
 * [puma-plugin-telemetry](https://github.com/babbel/puma-plugin-telemetry) - telemetry plugin for Puma offering various targets to publish
 
 ### Monitoring