How to make your raspberry pi a wifi router:
pc0---(via wifi)--\ pc1---(via wifi)---[wlan0 raspbery-pi eth0]---[ISP router]---internet pc2---(via wifi)--/
new manual: https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md
the new manual requires a kernel fix which is not released yet.
rpi-update; @see raspberrypi/linux#2437