docs: explain initrd network arguments usage

docs/usage/initrd-network-cmdline.md

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+# Initrd first-boot network arguments
+
+Afterburn supports injecting arguments for dracut-cmdline, in order to setup networking in initrd for the first boot.
+Valid network arguments are documented in [dracut manpages](https://www.man7.org/linux/man-pages/man7/dracut.cmdline.7.html).
+
+This is a feature with a fairly limited scope, which allows augmenting boot arguments ("kargs") in an additive-only way.
+Those arguments are injected by the `afterburn-network-kargs` service and processed by the network management stack in initrd (but they do not have effects on the kernel itself).
+
+If the kernel command-line already specifies some network arguments, Afterburn does not inject any additional parameters.
+Otherwise, distribution-specific values are inserted before the networking logic starts.
+
+On platforms where a suitable side-channel is available, it is possible for the user to override the defaults with customized values.
+
+# Platform-specific overrides
+
+## VMware
+
+On VMware, it is possible to provide a guestinfo property containing custom network arguments.
+Afterburn will lookup for the well-known key `guestinfo.afterburn.initrd.network-kargs` and use its value instead of the default.
+For this to work, the property must be set on the VM before the first boot. It does not affect subsequent boots.
+
+The example below shows a `node1` VM booting with a static hostname and IPv4 routing on the `ens192` interface.
+
+```
+VM_NAME="node1"
+
+IPCFG="ip=10.20.30.42::10.20.30.254:255.255.255.0:mynode01:ens192:off"
+
+govc vm.change -vm "${VM_NAME}" -e "guestinfo.afterburn.initrd.network-kargs=${IPCFG}"
+
+govc vm.power -on "${VM_NAME}"
+```