[Locale] updated README.md

src/Symfony/Component/Locale/README.md

@@ -20,3 +20,13 @@ Resources
 You can run the unit tests with the following command:
 
     phpunit
+
+If your PHP have the ``intl`` extension enabled but the intl extension ICU data
+version mismatch the one shipped with the component, you can build the data for
+it and use the ``USE_INTL_ICU_DATA_VERSION`` environment variable:
+
+    export USE_INTL_ICU_DATA_VERSION=true
+    phpunit
+
+Read the file ``Resources/data/UPDATE.txt`` for more info about building or
+updating the ICU data files.