docs: add cio import

Signed-off-by: Jess Valarezo <valarezo.jessica@gmail.com>

README.md

@@ -45,7 +45,11 @@ containerd offers a full client package to help you integrate containerd into yo
 
 ```go
 
-import "github.com/containerd/containerd"
+import (
+  "github.com/containerd/containerd"
+  "github.com/containerd/containerd/cio"
+)
+
 
 func main() {
 	client, err := containerd.New("/run/containerd/containerd.sock")