Add something about permissions in the Quick Tour

[wouterj]

In the Quick Tour, we show the installation process, but we don't say anything about permissions. This means people get confused when it doesn't work and have to search themselves for a solution, that's not good for the experience.

We should at last include a reference to http://symfony.com/doc/current/book/installation.html#book-installation-permissions imo.

Many thanks to @rodolfobandeira for explaining this issue to me on irc:

<ule> hey guys whatsup
<ule> I'd like to suggest something to put on documentation
<ule> I was trying to install symfony on my Amazon Ubuntu server and I had to change my /var/www/html folder to www-data group
<ule> and put my ubuntu default user to www-data
<ule> WouterJ: I didn't read nothing about folders write and read policies.
<ule> WouterJ: I get noticed about  writte cache error when I saw my /var/httpd/error.log
<WouterJ> ule: yeah, I confused it with the book installation chapter, where we do talk about permissions: http://symfony.com/doc/current/book/installation.html#book-installation-permissions
<ule> WouterJ: great! thank you
<ule> WouterJ: this installation link is much better than the quick tour

[wouterj]

ping @javiereguiluz