Add basic README explaining the intent around CDI injection

Signed-off-by: Evan Lezar <elezar@nvidia.com>

cmd/plugins/cdi-device-injector/README.md

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+# CDI Device Injector using NRI
+
+The purpose of this NRI plugin is to provide a controlled mechansim for injecting
+CDI devices into containers. This can be separated into two aspects:
+
+1. Requesting devices
+2. Controlling access to devices
+
+
+## Requesting devices
+
+For reqesting devices, we use pod annotations to indicate which devices should
+be made available to a particular container or containers in a pod. Here a
+pre-defined annotation prefix `cdi.nri.io/` is used for these annotations.
+Devices are requested for a specific container by including
+`container.{{ .ContainerName }}` as a suffix in the annotation key. For a
+request targeting ALL containers in a pod `cdi.nri.io/pod` is used as the pod
+annotaion key.
+
+In either case, the corresponding annotation value represents a comma-separated
+list of fully-qualified CDI device names.
+
+As examples, consider the following pod annotation:
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  namespace: management
+  name: nri-injection-example
+  annotations:
+    cdi.nri.io/container.first-ctr: "example.com/class=device0,example.com/class=device1"
+spec:
+  containers:
+  - name: first-ctr
+    image: ubuntu
+  - name: second-ctr
+    image: ubuntu
+```
+
+This will trigger the injection of the `example.com/class=device0` and
+`example.com/class=device1` devices into the `first-ctr` container, but not into
+the `second-ctr` container.
+
+When the annotations are updated as follows:
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  namespace: management
+  name: nri-injection-example
+  annotations:
+    cdi.nri.io/pod: "example.com/class=device0,example.com/class=device1"
+spec:
+  containers:
+  - name: first-ctr
+    image: ubuntu
+  - name: second-ctr
+    image: ubuntu
+```
+
+the same `example.com/class=device0` and `example.com/class=device1` devices
+will be injected into all (`first-ctr` and `second-ctr`) containers in the pod.
+
+## Controlling Access
+
+In order to control access to specific CDI devices, we make use of namespace
+annotations. Here, the same `cdi.nri.io/` prefix is used to identify an
+annotation for controlling the injection of CDI devices using NRI. The
+pre-defined annotation key `cdi.nri.io/allow` is used to explicitly allow access
+to CDI devices.
+
+The value field is interpreted as a filename glob to allow for wildcard matches.
+
+For example:
+* `*` will allow any CDI device to be injected
+* `example.com/*` will allow any CDI device with the explicit `example.com` to be
+  injected.
+* `example.com/class=*` will allow any CDI devices from vendor `example.com` and
+  class `class` to be injected.
+* `example.com/class=device0` will only allow the specified CDI device to be
+  injected.
+
+Consider the following example namespace (which was also referenced in the
+pod examples above):
+
+```yaml
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: management
+  annotations:
+    cdi.nri.io/allow: "*"
+```
+
+This allows the injection of any CDI devices into containers belonging to pods
+in this namespace.