note about on_worker_boot behavior when app is not preloaded (puma#2778)

* note about on_worker_boot behavior when app is not preloaded

puma#656 (comment)

* Prose

Co-authored-by: Nate Berkopec <nate.berkopec@gmail.com>

README.md

@@ -137,6 +137,11 @@ This code can be used to setup the process before booting the application, allow
 you to do some Puma-specific things that you don't want to embed in your application.
 For instance, you could fire a log notification that a worker booted or send something to statsd. This can be called multiple times.
 
+Constants loaded by your application (such as `Rails`) will not be available in `on_worker_boot`.
+However, these constants _will_ be available if `preload_app!` is enabled, either explicitly in your `puma` config or automatically if
+using 2 or more workers in cluster mode.
+If `preload_app!` is not enabled and 1 worker is used, then `on_worker_boot` will fire, but your app will not be preloaded and constants will not be available.
+
 `before_fork` specifies a block to be run before workers are forked:
 
 ```ruby