Here are instructions for setting up the raspi as a wifi access point: https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md
Best to do this using wired access!
If you want the raspi to auto-detect your home wifi network and use that if available, and become a wifi access point only if your home wifi network is unavailable, the instructions here work: http://www.raspberryconnect.com/network/item/315-rpi3-auto-wifi-hotspot-if-no-internet
Caveat: I did this on a raspi 3, and using Raspian, not the Nerves-installed OS.
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