Add screenshot of XPI exe

[diamondo25]

The executable included inside the XPI file has nothing special (except 'Congratulation!')
I've added the window you get when running the EXE (I've checked the binary with dotPeek; nothing special is done). The executable has been changed on 10-9-2010 at 15:59, however.

[aspensmonster]

Good to know. You ought to update the README.md in the xpi/ directory with your findings.