Added missing links

Author: javiereguiluz

setup.rst

@@ -268,3 +268,10 @@ Learn More
 .. _`Main recipe repository`: https://github.com/symfony/recipes
 .. _`Contrib recipe repository`: https://github.com/symfony/recipes-contrib
 .. _`Symfony Recipes documentation`: https://github.com/symfony/recipes/blob/master/README.rst
+.. _`iconv`: https://php.net/book.iconv
+.. _`JSON`: https://php.net/book.json
+.. _`Session`: https://php.net/book.session
+.. _`Ctype`: https://php.net/book.ctype
+.. _`Tokenizer`: https://php.net/book.tokenizer
+.. _`SimpleXML`: https://php.net/book.simplexml
+.. _`PCRE`: https://php.net/book.pcre