documentation: add configuration instructions

Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>

Author: bart0sh

README.md

@@ -20,6 +20,36 @@ 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
+
+### 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.
+
 ## Examples
 ```bash
 $ mkdir /etc/cdi