Added readme

Author: Nyholm

Readme.md

@@ -72,6 +72,29 @@ class BundleInitializationTest extends BaseBundleTestCase
 
 ```
 
+## Private services in Symfony 4
+
+In Symfony 4 services are private by default. This is a good thing, but in order to test them properly we need to make
+them public when we are running the tests. This can easily be done with a compiler pass. 
+
+```php
+class BundleInitializationTest extends BaseBundleTestCase
+{
+    protected function setUp()
+    {
+        parent::setUp();
+        
+        // Make all services public
+        $this->addCompilerPass(new PublicServicePass());
+        
+        // Make services public that have an idea that matches a regex
+        $this->addCompilerPass(new PublicServicePass('|my_bundle.*|'));
+    }
+
+    // ...
+}
+```
+
 ## Configure Travis
 
 You want Travis to run against each currently supported LTS version of Symfony (since there would be only one per major version), plus the current if it's not an LTS too. There is no need for testing against version in between because Symfony follows [Semantic Versioning](http://semver.org/spec/v2.0.0.html).