Update README for airgap documentation

README.md

@@ -397,6 +397,26 @@ for port 80.  If no port is available the load balancer will stay in Pending.
 To disable the embedded service load balancer (if you wish to use a different implementation like
 MetalLB) just add `--no-deploy=servicelb` to the server on startup.
 
+Air-Gap Support
+---------------
+
+k3s supports pre-loading of containerd images by placing them in the `images` directory for the agent before starting, eg:
+```sh
+sudo mkdir -p /var/lib/rancher/k3s/agent/images/
+sudo cp ./k3s-airgap-images-$ARCH.tar /var/lib/rancher/k3s/agent/images/
+```
+Images needed for a base install are provided through the releases page, additional images can be created with the `docker save` command.
+
+Offline Helm charts are served from the `/var/lib/rancher/k3s/server/static` directory, and Helm chart manifests may reference the static files with a `%{KUBERNETES_API}%` templated variable. For example, the default traefik manifest chart installs from `https://%{KUBERNETES_API}%/static/charts/traefik-X.Y.Z.tgz`.
+
+If networking is completely disabled k3s may not be able to start (ie ethernet unplugged or wifi disconnected), in which case it may be necessary to add a default route. For example:
+```sh
+sudo ip -c address add 192.168.123.123/24 dev eno1
+sudo ip route add default via 192.168.123.1
+```
+
+k3s additionally provides a `--resolv-conf` flag for kubelets, which may help with configuring DNS in air-gap networks.
+
 TODO
 ----
 Currently broken or stuff that needs to be done for this to be considered production quality.