Update Event guides

Documenting automatic loading of files and subscription of subscribers
when their files are placed under the directory `app/subscribers` and
end with `_subscriber.rb`.

guides/source/developers/events/overview.html.md

@@ -95,10 +95,18 @@ method allows to map a method of the subscriber module to an event that
 happens in the system. If the `event_name` argument is not specified,
 the event name and the method name should match.
 
-These subscribers modules are loaded with the rest of your application but
-you need to manually subscribe to them.
-
-For example, you could subscribe to them programmatically with something like:
+These subscriber modules are loaded with the rest of your application, you
+don't need to manually subscribe them when:
+
+* `Spree::Config.events.autoload_subscribers` returns a truthy value (default);
+* you put them in the directory (or any subdirectory of) `app/subscribers`;
+* their filename ends with `_subscriber.rb`;
+
+On the other hand, if you need to resort to manual subscription because you did
+not follow the naming convention explained above, or you prefer to have complete
+control on what is loaded and when, you can override the default behaviour by
+setting `Spree::Config.events.autoload_subscribers = false` in a initializer.
+At that point you can subscribe your event subscribers, with something similar to:
 
 ```ruby
 if defined?(SmsLibrary)