bug #58 Update custom listener documentation (maciekpaprocki)

This PR was merged into the 1.8-dev branch.

Discussion
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| Q               | A
| --------------- | -----
| Branch?         | any since the implementation
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | just doc
| License         | MIT

The current doc is a bit outdated. 

You need to implement the right interface to tell the system which hook should be run.  

Hopefully, that will help someone in the future trying to figure out why their listener is not working.

Commits
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2ec967b Update custom listener documentation

docs/custom_listener.md

@@ -26,7 +26,13 @@ final class DirectoryPurgerListener extends AbstractListener implements BeforeSu
 }
 ```
 
-The next step is to register this listener:
+If you want to listen to different/more event(s) instead of BeforeSuiteListenerInterface implement one or more of those interfaces:
+- AfterFixtureListenerInterface
+- AfterSuiteListenerInterface
+- BeforeFixtureListenerInterface
+- BeforeSuiteListenerInterface
+
+The next step is to register this listener ( you can avoid this step when using autoconfiguration ):
 
 ```xml
 <service id="app.listener.directory_purger" class="App\Fixture\DirectoryPurgerListener">