update example device readme (#24124)

plugins/device/cmd/example/README.md

@@ -5,15 +5,31 @@ reference.
 
 The example device plugin models files within a specified directory as devices. The plugin will periodically scan the directory for changes and will expose them via the streaming Fingerprint RPC. Device health is set to unhealthy if the file has a specific filemode permission as described by the config `unhealthy_perm`. Further statistics are also collected on the detected devices.
 
+# Installation
+
+```shell
+nomad_plugin_dir='/opt/nomad/plugins' # for example
+go build -o $nomad_plugin_dir/nomad-device-example ./cmd
+```
+
 # Config
 
-The configuration should be passed via an HCL file that begins with a top level `config` block:
+Example client agent config with our
+[plugin](https://developer.hashicorp.com/nomad/docs/configuration/plugin) block:
 
-```
+```hcl
-config {
+client {
-  dir = "/my/path/to/scan"
+  enabled = true
-  list_period = "1s"
+}
-  unhealthy_perm = "-rw-rw-rw-"
+
+plugin_dir = "/opt/nomad/plugins"
+
+plugin "nomad-device-example" {
+  config {
+    dir            = "/tmp/nomad-device"
+    list_period    = "1s"
+    unhealthy_perm = "-rwxrwxrwx"
+  }
 }
 ```
 
@@ -22,3 +38,84 @@ The valid configuration options are:
 * `dir` (`string`: `"."`): The directory to scan for files that will represent fake devices.
 * `list_period` (`string`: `"5s"`): The interval to scan the directory for changes.
 * `unhealthy_perm` (`string`: `"-rwxrwxrwx"`): The file mode permission that if set on a detected file will casue the device to be considered unhealthy.
+
+# Usage
+
+Create two instances of the device, one unhealthy:
+
+```shell
+mkdir -p /tmp/nomad-device
+cd /tmp/nomad-device
+touch device01 && chmod 0777 device01
+touch device02
+```
+
+It should be fingerprinted by the client agent after the `list_period`,
+which you can check with:
+
+```shell
+nomad node status -json -self | jq '.NodeResources.Devices'
+```
+
+```json
+[
+  {
+    "Attributes": null,
+    "Instances": [
+      {
+        "HealthDescription": "Device has bad permissions \"-rwxrwxrwx\"",
+        "Healthy": false,
+        "ID": "device01",
+        "Locality": null
+      },
+      {
+        "HealthDescription": "",
+        "Healthy": true,
+        "ID": "device02",
+        "Locality": null
+      }
+    ],
+    "Name": "mock",
+    "Type": "file",
+    "Vendor": "nomad"
+  }
+]
+
+```
+
+The value to put in job specification
+[device](https://developer.hashicorp.com/nomad/docs/job-specification/device)
+block, or a quota specification,
+is `"{Vendor}/{Type}/{Name}"` i.e. `"nomad/file/mock"`:
+
+`job.nomad.hcl`:
+
+```hcl
+job "job" {
+  group "grp" {
+    task "tsk" {
+      driver = "..."
+      config {}
+      resources {
+        device "nomad/file/mock" {
+          count = 1
+        }
+      }
+    }
+  }
+}
+```
+
+`dev.quota.hcl`:
+
+```hcl
+name = "dev"
+limit {
+  region = "global"
+  region_limit {
+    device "nomad/file/mock" {
+      count = 2 # to allow for deployments/reschedules
+    }
+  }
+}
+```