Add example of desktop file

examples/Cadbiom.desktop

+[Desktop Entry]
+Comment=Cadbiom GUI launcher
+Encoding=UTF-8
+Exec=<path_to_your_home>/.virtualenvs/<virtualenv_name>/bin/cadbiom
+Icon=<path_to_your_home>/.virtualenvs/<virtualenv_name>/lib/python2.7/site-packages/cadbiom_gui/gt_gui/images/favicon.ico
+Categories=Science;
+Name=Cadbiom
+Type=Application
+Terminal=false