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)--/
old manual: https://raspberrypihq.com/how-to-turn-a-raspberry-pi-into-a-wifi-router/
old extra tip: https://www.raspberrypi.org/forums/viewtopic.php?t=191678
new manual: https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md
the new manual requires a kernel fix which is not released yet.
so, do rpi-update
; @see raspberrypi/linux#2437