Create additional presentation with some extra information.
On branch master, changes to be committed:
	new file:   Linux_intro_extra.odp
	modified:   Linux_intro_presentation.odp
	new file:   desktop_environment_images/Gnome/GNOME_3_classic_mode.png
	new file:   desktop_environment_images/Gnome/Gnome3-Fallback-Mode.png
	new file:   desktop_environment_images/Gnome/Gnome_3.6_on_Arch.png
	new file:   desktop_environment_images/Gnome/Gnomelogo.svg
	new file:   desktop_environment_images/KDE/KDE_4.png
	new file:   desktop_environment_images/KDE/KDE_logo.svg
	new file:   desktop_environment_images/LXDE/LXDE-logo.svg
	new file:   desktop_environment_images/LXDE/LXDE_desktop_full.png
	new file:   desktop_environment_images/LXDE/Lxpanel_menu.png
	new file:   desktop_environment_images/LXDE/Pcmanfm.png
	new file:   desktop_environment_images/Mate/Linux_Mint_17_MATE.png
	new file:   desktop_environment_images/Unity/Screenshot_from_2014-04-25_21_06_50.png
	new file:   desktop_environment_images/Unity/Unity_logo.svg
	new file:   desktop_environment_images/XFCE/XFCE-4.10-Desktop.png
	new file:   desktop_environment_images/XFCE/Xfce-4.4.png
	new file:   desktop_environment_images/XFCE/Xfce3.jpg
	new file:   desktop_environment_images/XFCE/Xfce_logo.svg
	new file:   desktop_environment_images/sources_and_credits.md