doc: update doc for ignition flag usage

it updates the doc to explain how the ignition flag is used. It also adds a new section under contrib where to store an example of configuration and link to the ignition website for more details about specification, mime type and other examples.

contrib/ignition/README.md

@@ -0,0 +1,9 @@
+## Ignition
+
+Ignition uses a JSON configuration file to define the desired changes. The format of this config is specified in detail [here](https://coreos.github.io/ignition/specs/), and its [MIME type](http://www.iana.org/assignments/media-types/application/vnd.coreos.ignition+json) is registered with IANA.
+
+`myconfig.json` file provides an example of configuration that adds a new `testuser`, creates a new file to `/etc/myapp` with the content listed in the same `files` section and add a systemd unit drop-in to modify the existing service `systemd-journald` and sets its environment variable SYSTEMD_LOG_LEVEL to debug.
+
+### Examples
+
+More examples can be found at https://coreos.github.io/ignition/examples/

contrib/ignition/myconfig.json

@@ -0,0 +1,31 @@
+{
+  "ignition": {
+    "version": "3.0.0"
+  },
+  "passwd": {
+    "users": [
+      {
+        "name": "testuser",
+        "sshAuthorizedKeys": [
+          "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIO43Z9aRPvvj1zMib/Hh4oWX+MuOPXdFChFMaWfishLj"
+        ]
+      }
+    ]
+  },
+  "storage": {
+    "files": [{
+      "path": "/etc/someconfig",
+      "mode": 420,
+      "contents": { "source": "data:,example%20file%0A" }
+    }]
+  },
+  "systemd": {
+    "units": [{
+      "name": "systemd-journald.service",
+      "dropins": [{
+        "name": "debug.conf",
+        "contents": "[Service]\nEnvironment=SYSTEMD_LOG_LEVEL=debug"
+      }]
+    }]
+  }
+}

doc/usage.md

@@ -460,3 +460,18 @@ Proper use of this flag may look similar to the following section of a command:
 ```bash
 --device virtio-input,keyboard --device virtio-input,pointing --device virtio-gpu,width=1920,height=1080 --gui
 ```
+
+### Ignition
+
+#### Description
+
+The `--ignition` option allows you to specify a configuration file for Ignition. Vfkit will open a vsock connection between the host and the guest and start a lightweight HTTP server to push the configuration file to Ignition.
+
+You can find example configurations and more details about Ignition at https://coreos.github.io/ignition/ 
+
+#### Example
+
+This command provisions the configuration file to Ignition on the guest
+```
+--ignition configuration-path
+```