Added instructions for Xfce and Openbox DE
Added instructions for Xfce and Openbox DE

I think it would be great to add info about these DE's in extension description.

Thanks!
carlin-q-scott/browser-media-keys,gmelikov/browser-media-keys,miicha/browser-media-keys,gmelikov/browser-media-keys,miicha/browser-media-keys,gmelikov/browser-media-keys,carlin-q-scott/browser-media-keys