How?

Stick on glass door with window decals

Why?
Can be done on the outside, the inside or even from the inside facing out.
cut option: contour cut (clear cut with no borders)
More research on window decals:
[http://www.signs.com/blog/window-signage-options-signage-101/]


Method to remove the sticker: 
http://www.wikihow.com/Remove-a-Sticker-from-Glass