More doc

README.md

@@ -210,6 +210,31 @@ Next step: qmi-utils!
  - https://community.autopi.io/t/error-when-starting-qmi-connection/2338
  - https://lists.freedesktop.org/archives/libqmi-devel/2018-January/002707.html
 
+## ModemManager
+
+We append the following to ModemManager:
+
+```
+config interface 'broadband'
+        option device '/sys/devices/platform/soc/60f8800.usb2/6000000.dwc3/xhci-hcd.1.auto/usb3/3-1'
+        option proto 'modemmanager'
+        option apn 'free'
+        option iptype 'ipv4v6'
+```
+
+Then we can start the interface with:
+
+```
+ifup broadband
+```
+
+Both IPv6 and IPv4 should work:
+
+```
+ping openwrt.org
+ping -6 openwrt.org
+```
+
 ## Logs
 
 uboot logs: