feature symfony#5060 Adds note on new validation files scanned in 2.7 (GromNaN)

This PR was merged into the 2.7 branch.

Discussion
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Adds note on new validation files scanned in 2.7

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#13855)
| Applies to    | 2.7
| Fixed tickets | -

Commits
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0967d07 Adds note on new validation files scanned in 2.7

Author: wouterj

book/validation.rst

@@ -103,6 +103,11 @@ following:
     Protected and private properties can also be validated, as well as "getter"
     methods (see :ref:`validator-constraint-targets`).
 
+.. versionadded:: 2.7
+    As of Symfony 2.7, XML and Yaml constraint files located in the
+    ``Resources/config/validation`` sub-directory of a bundle are loaded. Prior
+    to 2.7, only ``Resources/config/validation.yml`` (or ``.xml``) were loaded.
+
 .. index::
    single: Validation; Using the validator