First install the qrencode utility:

# apt-get install -y qrencode

You can see the version:

# qrencode -V
qrencode version 3.1.1
Copyright (C) 2006, 2007, 2008, 2009 Kentaro Fukuchi

Now create the QR code with this string:

# qrencode -o wifi.png "WIFI:S:fordodone-wifi-ssid;T:WPA2;P:F0rd0d0n3;;"

The pattern is “WIFI:S:<yournetworkssid>;T:WPA2;P:<wpa2passphrase>;;” Be sure to note the semicolons vs. colons.