Change potentially system-wrecking pacman command in README.

The README had a pacman command (`-Syy`, to be specific), which could break one's system if not used carefully.

A guy named "Allan" explained why in this [BBS thread](https://bbs.archlinux.org/viewtopic.php?id=89328).

>It can create dependency issues as Arch is designed only to have the latest versions of packages installed. e.g. enable testing and install the latest firefox with "pacman -Sy firefox"...  that will pull in the new versions of libjpeg and libpng which will break a lot of your applications that still require the old version.   Updating all packages avoids that issue.
Issues with graphical packages is not so bad, but people got themselves into major trouble when updating readline (broken bash...)