Add Symfony integration explanation

Author: iquito

README.md

@@ -22,6 +22,30 @@ $twig->addExtension(new \Squirrel\TwigPhpSyntax\PhpSyntaxExtension());
 
 You can also have a look at the extension definition and create your own extension class to only include some of the features, if you do not like all of them.
 
+### Symfony integration
+
+If you use `autoconfigure` (which is the default) you just need to load the PhpSyntaxExtension class in `services.yaml` in the `config` directory of your project (the first four lines should already be there, just add the line with the PhpSyntaxExtension class at the end of the file):
+
+```yaml
+services:
+    _defaults:
+        autowire: true
+        autoconfigure: true
+
+    # Just add the following line, Symfony will register
+    # the extension in Twig for you if Twig is installed
+    Squirrel\TwigPhpSyntax\PhpSyntaxExtension: ~
+```
+
+If you do not use `autoconfigure`, you can add the twig extension tag to the service definition:
+
+```yaml
+services:
+    Squirrel\TwigPhpSyntax\PhpSyntaxExtension:
+        tags:
+            - { name: twig.extension }
+```
+
 Features
 --------