Merge pull request #67 from bart0sh/PR0013-config-documentation

documentation: add configuration instructions

Author: elezar

README.md

@@ -20,6 +20,52 @@ However, as devices and software grows more complex, vendors want to perform mor
 In the absence of a standard for third party devices, vendors often have to write and maintain multiple plugins for different runtimes or even directly contribute vendor specific code in the runtime.
 Additionally runtimes don't uniformly expose a plugin system (or even expose a plugin system at all) leading to duplication of the functionality in higher level abstractions (such as Kubernetes device plugins).
 
+## How does CDI work?
+
+For CDI to work the following needs to be done:
+
+- CDI file containing update for the OCI spec in JSON format should be present in the CDI
+  spec directory. Default directories are /etc/cdi and /var/run/cdi
+
+- Fully qualified device name should be passed to the runtime either
+  using command line parameters for podman or using container annotations
+  for CRI-O and Containerd
+
+- Container runtime should be able to find CDI file by the device name
+  and update container config using CDI file content.
+
+## How to configure CDI?
+
+### CRI-O configuration
+
+In CRI-O CDI support is enabled by default. It is configured with the default `/etc/cdi, /var/run/cdi`
+CDI directory locations. Therefore you can start using CDI simply by dropping CDI configuration files
+in either of those directories, static configuration into `/etc/cdi` and dynamically updated one into
+`/var/run/cdi`. If you are unsure of the configured directories you can run this command to find them
+out:
+
+```bash
+$ crio config |& grep -B1 -A5 cdi_spec_dirs
+```
+
+### Containerd configuration
+
+To enable and configure CDI support in the [containerd runtime](https://github.com/containerd/containerd) 2 configuration options `enable_cdi` and `cdi_spec_dirs` should be set in the `plugins."io.containerd.grpc.v1.cri` section of the containerd configuration file (`/etc/containerd/config.toml` by default):
+
+```
+[plugins."io.containerd.grpc.v1.cri"]
+  enable_cdi = true
+  cdi_spec_dirs = ["/etc/cdi", "/var/run/cdi"]
+```
+
+Remember to restart containerd for any configuration changes to take effect.
+
+### Podman configuration
+
+[podman](https://github.com/containers/podman) does not require any specific configuration to enable CDI support and processes specified `--device` flags directly. If fully-qualified device selectors (e.g. `vendor.com/device=myDevice`) are included the CDI specifications at the default location (`/etc/cdi` and `/var/run/cdi`) are checked for matching devices.
+
+*Note:* Although initial support was added in [`v3.2.0`](https://github.com/containers/podman/releases/tag/v3.2.0) this was updated for the tagged `v0.3.0` CDI spec in [`v4.1.0-rc.1`](https://github.com/containers/podman/releases/tag/v4.1.0-rc1) with [commit a234e4e](https://github.com/containers/podman/commit/a234e4e19662e172472877ce69523f4afea5c12e).
+
 ## Examples
 ```bash
 $ mkdir /etc/cdi