Add a note that puma-plugin-systemd is redundant now (#3120)

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. Puma 5.1.0 integrated notify and watchdog, which probably conflicts with this plugin. Puma 6.1.0 added status support which obsoletes the plugin entirely.
 * [puma-plugin-telemetry](https://github.com/babbel/puma-plugin-telemetry) - telemetry plugin for Puma offering various targets to publish
 
 ### Monitoring