Adding shower engine as git submodule
Updating description
Adding Linux distributions images

https://upload.wikimedia.org/wikipedia/commons/4/4a/Debian-OpenLogo.svg
https://upload.wikimedia.org/wikipedia/commons/e/eb/Official_Linux_Mint_logo.svg
https://upload.wikimedia.org/wikipedia/en/a/ac/Archlinux-official-fullcolour.svg

https://upload.wikimedia.org/wikipedia/en/9/98/OpenSUSE_official-logo-color.svg
https://upload.wikimedia.org/wikipedia/commons/3/3a/Logo-ubuntu_no%28r%29-black_orange-hex.svg
https://upload.wikimedia.org/wikipedia/en/9/94/Fedora_Project_logo.png
https://upload.wikimedia.org/wikipedia/en/6/6c/RedHat.svg
https://upload.wikimedia.org/wikipedia/en/a/a9/CentOS_full_logo.svg
https://upload.wikimedia.org/wikipedia/commons/4/48/Gentoo_Linux_logo_matte.svg

https://upload.wikimedia.org/wikipedia/commons/3/34/Slackware_web_logo.svg
Linux GUI screenshots

https://upload.wikimedia.org/wikipedia/commons/4/48/GNOME_3.6_with_Shell_Overview.png
https://upload.wikimedia.org/wikipedia/commons/5/54/KDE_4.png
https://upload.wikimedia.org/wikipedia/commons/0/06/Unity_5.12_on_Ubuntu_12.04.png
https://upload.wikimedia.org/wikipedia/commons/8/87/Cinnamon_1.6.7%2C_Menu_%28Linux_Mint_Maya%29.png
https://upload.wikimedia.org/wikipedia/commons/d/d7/XFCE-4.10-Desktop.png
GUI icons

https://upload.wikimedia.org/wikipedia/commons/9/90/X.Org_Logo.svg
https://upload.wikimedia.org/wikipedia/commons/6/68/Gnomelogo.svg
https://upload.wikimedia.org/wikipedia/commons/5/5b/Xfce_logo.svg
https://upload.wikimedia.org/wikipedia/commons/8/8d/KDE_logo.svg
https://upload.wikimedia.org/wikipedia/commons/9/99/Wayland_Logo.svg
History, Graphics (incomplete)
http://www.linuxshowcase.org/1999/images/davinci.gif
https://upload.wikimedia.org/wikipedia/en/2/22/Heckert_GNU_white.svg
https://upload.wikimedia.org/wikipedia/commons/3/35/Tux.svg
https://upload.wikimedia.org/wikipedia/commons/0/0a/Unity_Logo.svg
Software slides
Sudo make me a sandwich
Adding the shower engine
Advanced linux