[VarDumper] add a tip about global dump() availability

Author: nicolas-grekas

components/var_dumper/introduction.rst

@@ -62,6 +62,17 @@ current PHP SAPI:
     You'll also learn how to change the format or redirect the output to
     wherever you want.
 
+.. tip::
+
+    In order to have the ``dump()`` function always available, you can install
+    it globally in your dev environment:
+
+    #. Run ``composer global require symfony/var-dumper``;
+    #. Add ``auto_prepend_file = ${HOME}/.composer/vendor/autoload.php``
+       to your ``php.ini`` file;
+    #. (Don't forget to run ``composer global update`` from time to time to always
+       have the latest bug fixes.)
+
 DebugBundle and Twig Integration
 --------------------------------