minor #6328 Update extension.rst - added caution box for people trying to remove the default file with services definitions (Pavel Jurecka)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6328).

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

Following steps from the original article isn't enough, because config.yml still tries to import services.yml from the original location.

Commits
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8369fe5 Update extension.rst - added caution box for people trying to remove the default file with services definitions

Author: xabbuh

cookbook/bundles/extension.rst

@@ -113,6 +113,12 @@ Other available loaders are the ``YamlFileLoader``, ``PhpFileLoader`` and
     The ``IniFileLoader`` can only be used to load parameters and it can only
     load them as strings.
 
+.. 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``.
+
 Using Configuration to Change the Services
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~