Update extension.rst - added caution box for people trying to remove the default file with services definitions

Author: Pavel Jurecka

cookbook/bundles/extension.rst

@@ -119,3 +119,8 @@ Using Configuration to Change the Services
 The Extension is also the class that handles the configuration for that
 particular bundle (e.g. the configuration in ``app/config/config.yml``). To
 read more about it, see the ":doc:`/cookbook/bundles/configuration`" article.
+
+.. caution::
+
+    If you removed the default file with services definitions (i. e. ``app/config/services.yml``),
+    make sure to also remove it from the ``imports`` key in ``app/config/config.yml``.