Create a file in the root of boot called: wpa_supplicant.conf (instructions below). Then paste the following into it (adjusting for your ISO 3166 alpha-2 country code, network name and network password):
country=US
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
network={
ssid="NETWORK-NAME"
psk="NETWORK-PASSWORD"
}
# for raspberry pi
wpa_passphrase 'MyHomeWireless' 'MyHomePassword' >> /etc/wpa_supplicant/wpa_supplicant.conf