Added documentation for serializer config

- Added documenation for the serializer config files under the root project directory Signed-off-by: RJ Garcia <rj@bighead.net>

serializer.rst

@@ -136,7 +136,7 @@ to your class and choose which groups to use when serializing::
 .. tip::
 
  The value of the ``groups`` key can be a single string, or an array of strings.
- 
+
  .. versionadded:: 4.2
  The option to pass a single string to ``groups`` was introduced in Symfony 4.2.
 
@@ -148,6 +148,8 @@ stored in one of the following locations:
  the ``Resources/config/`` directory of a bundle;
 * All ``*.yaml`` and ``*.xml`` files in the ``Resources/config/serialization/``
  directory of a bundle.
+* All ``*.yaml`` and ``*xml`` files in the ``config/serializer`` directory in the
+ project directory.
 
 .. _serializer-enabling-metadata-cache: