added desktop file

This commit adds a '.desktop' file, to be used by Debian when installing
with apt

 * https://specifications.freedesktop.org/desktop-entry/latest/

For more info, see:

 * #103

Author: maltfield

src/buskill.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Name=BusKill
+Comment=Laptop Kill Cord
+Exec=/usr/bin/buskill
+Terminal=false
+Type=Application
+Categories=System;Security
+Keywords=buskill;kill cord;killcord;security;lock;shutdown;wipe;encrypt;emergency;dead man switch
+