add documentation for running VM with bridged network in NAT mode

how-to-run-a-vm-with-nat-bridged-network.md

@@ -0,0 +1,47 @@
+create a bridge configuration file, /tmp/virbro.xml :  
+```xml
+<network>
+  <name>default</name>
+  <uuid>9014424a-8422-40ec-94d1-9f593b98cb7a</uuid>
+  <forward mode="nat"/>
+  <bridge name="virbr0" stp="on" delay="0"/>
+  <mac address="52:54:00:ed:60:11"/>
+  <ip address="192.168.122.1" netmask="255.255.255.0">
+    <dhcp>
+      <range start="192.168.122.2" end="192.168.122.254"/>
+    </dhcp>
+  </ip>
+</network>
+```
+
+apply bridge configuration :  
+```sh
+virsh net-define /tmp/virbr0.xml
+virsh net-start virbr0
+```
+
+optionnaly, make it to auto start :  
+```sh
+virsh net-autostart virbr0
+```
+
+
+copy the VM start script 
+```sh
+cp /nix/store/iim2hsbfj7kbxfygn741ica52fka6xk3-nixos-vm/bin/run-hedgedoc-vm ./run-vm-2
+```
+
+change network options : 
+```sh
+<     -net nic,netdev=user.0,model=virtio -netdev user,id=user.0,"$QEMU_NET_OPTS" \
+---
+>     -net nic,netdev=user.0,model=virtio -netdev bridge,br=virbr0,id=user.0,"$QEMU_NET_OPTS" \
+```
+
+
+
+run the VM : 
+```sh
+QEMU_NET_OPTS=helper=/run/wrappers/bin/qemu-bridge-helper ./run-vm-2
+```
+