Add doctrine bundle in dependencies

This is a requirement in order to make the Symfony Recipe work. Of
course it's sad because the library will enforce the installation of the
doctrine bundle. But at this moment I don't think anybody installed this
library without Symfony and the Doctrine bundle. Also the bundle present
in our bridge is dependent of the Symfony Doctrine Bundle.

Taking all of this, I think it's not a bad thing and we will reconsider
if it's a problem some day.

Notice that it also contains doctrine orm requirement, which is not
contained by the doctrine bundle.

composer.json

@@ -4,7 +4,9 @@
     "license": "MIT",
     "require": {
         "php": ">=7.1",
-        "symfony/event-dispatcher": "~3.3|~4.0"
+        "symfony/event-dispatcher": "~3.3|~4.0",
+        "doctrine/doctrine-bundle": "^1.8",
+	"doctrine/orm": "~2.3"
     },
     "authors": [
         {