Improved Linux install process

The documentation tells Linux users to unpack the archive in their /home directory and install Processing in that location. While this works (and is shorter), it might be preferable to install the software in the /opt directory. Not only is /opt the proper location to install so-called optional software, but launch shortcuts will aslo be added to the panel and desktop.

Description of install process in /opt directory :

1. Download the file to your /home directory.
2. Open a terminal window, and type tar xvfz processing-xxxx.tgz (replace xxxx with the rest of the file's name, which is the version number.) This will extract the content of the archive and create a folder named processing-4.0 or something similar.
3. Move the processing folder to the /opt directory using this command sudo mv processing-xxxx /opt/
4. Navigate to its location cd /opt/processing-xxxx/
5. Perform the installation ./install.sh

Apri la documentazione introduttiva collegata e individua le istruzioni per l'installazione su Linux. Aggiornale per descrivere chiaramente il workflow /opt proposto, quindi verifica la pagina renderizzata e i comandi di installazione.