docs: document configmode

docs/features/configmode.rst

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+Config Mode
+===========
+
+When in Config Mode a node will neither participate in the mesh nor connect
+to the VPN using the WAN port. Instead, it'll offer a web interface on the
+LAN port to aid configuration of the node.
+
+Whether a node is in Config Mode can be determined by a characteristic
+blinking sequence of the SYS LED:
+
+.. image:: node_configmode.gif
+
+Activating Config Mode
+----------------------
+
+Config Mode is automatically entered at the first boot. You can re-enter
+Config Mode by pressing and holding the RESET/WPS button for about three
+seconds. The device should reboot (all LEDs will turn of briefly) and 
+Config Mode will be available.
+
+
+Port Configuration
+------------------
+
+In general, Config Mode will be offered on the LAN ports. However, there
+are two practical exceptions:
+
+* Devices with just one network port will run Config Mode on that port.
+* Devices with PoE on the WAN port will run Config Mode on the WAN port instead.
+
+
+Accessing Config Mode
+---------------------
+
+Config Mode can be accessed at http://192.168.1.1. The node will offer DHCP
+to clients. Should this fail, you may assign an IP from 192.168.1.0/24 to
+your computer manually.