Add a README.md on dind with minimal information

Signed-off-by: Manuel Buil <mbuil@suse.com>

docs/dind.md

@@ -0,0 +1,47 @@
+# rke DinD
+
+Docker-in-Docker (DinD) feature is created mainly for testing.
+
+DinD creates Docker containers locally which are then used as cluster nodes.
+
+It still requires a cluster.yml with minimal information:
+
+```
+kubernetes_version: "$KUBE_VERSION"
+nodes:
+- address: foo
+  role:
+  - controlplane
+  - worker
+  - etcd
+  user: bar
+```
+where KUBE_VERSION is any version in the output list of the command `rke config --list-version --all`. Note that in multinode clusters, the value of address must be unique
+
+To create a cluster with DinD execute:
+`rke up --dind`
+
+After the creation is completed, you will see one Docker container per node running. You can use the created `kube_config_cluster.yml` to access the cluster as in a normal RKE deployment
+
+To remove a cluster with DinD execute:
+`rke remove --dind`
+
+The Docker containers should be removed completely
+
+# FAQ
+
+* Do I need to create a network for the Docker containers?
+
+No. The created Docker containers use the default Docker bridge network which allows to connect from the host to the containers on IPs in the subnet "172.17.0.0/16". However, they have no ssh service running, therefore you `docker exec` if you want to connect to them
+
+* What image is used for those containers?
+
+The default Docker dind image. You can find the used tag in this [link](https://github.com/rancher/rke/blob/release/v1.5/dind/dind.go#L16)
+
+* Are all RKE features supported in DinD?
+
+No. Only creation and removal of clusters are supported. You can of course operate the Kubernetes cluster as a normal cluster
+
+* Is DinD production ready?
+
+No. DinD's purpose is solely RKE testing. Anything else is out of scope