Add install note for short_open_tag (PHP <5.4 compatability)

None of the templates render correctly with the `short_open_tag` PHP directive disabled for PHP versions earlier than 5.4. This behavior changed in 5.4, so it won't be a problem for everyone (or for much longer).

See http://us3.php.net/manual/en/ini.core.php#ini.short-open-tag