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

Author: nicolas-grekas

components/var_dumper/introduction.rst

@@ -62,6 +62,15 @@ 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 on your dev environment. Just follow these steps: run
+    ``composer global require symfony/var-dumper`` then 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 keep up with the latest fixes.
+
 DebugBundle and Twig Integration
 --------------------------------