Adds Bridge_Mode doc

docs/Bridge_Mode.md

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+# Bridge Mode
+
+A common feature of COTS Wi-Fi routers is to be able to set the router
+to bridge mode, where the router stops acting like a router (layer 3
+device) and starts acting like an ethernet switch (layer 2
+device). LibreCMC does not actually have a bridge mode, but the same
+effect is achievable with a few easy steps.
+
+1. Add the LAN ethernet port to the LAN bridge interface
+2. Set the DHCP server to ignore the LAN interface
+
+## Adjust LAN bridge interface
+
+Go to Network >> Interfaces page and press the EDIT button next to LAN
+Network. Select the Physical Settings tab.
+
+![alt text](images/librecmc-default-interfaces.png "LibreCMC default
+ interfaces view")
+
+Place a checkmark in the box for the "eth0" interface for wan and
+wan6. (I suppose the interface name might vary amongst different
+routers.
+
+![alt text](images/librecmc-eth0-added-to-lan.png "Adjustments to
+ LibreCMC interface physical settings")
+
+You can also check the box for Enable STP if desired. There doesn't
+seem to be any downside from this, though I suppose it would use more
+CPU cycles and memory. STP is a protocol intended to prevent
+communication loops from forming in a network of layer 2 switches.
+
+## Adjust DHCP server settings
+
+On the same page, scroll down a little to the DHCP Server section. In
+the General Setup tab, check the "Ignore interface" box.