Update deploy-env role README.md

Change-Id: Ia2ace3541be97577f1225d54417f6a287b7a8eb2

roles/deploy-env/README.md

@@ -5,8 +5,22 @@ This role is used to deploy test environment which includes
 - install Calico as a Kubernetes networking
 - establish tunnel between primary node and K8s control plane ndoe
 
-The role works both for singlenode and multinode inventories and
+The role works both for single-node and multi-node inventories. The role
-assumes the inventory has the node called `primary` and the group called `nodes`.
+totally relies on inventory groups. The `primary` and `k8s_control_plane`
+groups must include only one node and this can be the same node for these two
+groups.
+
+The `primary` group is where we install `kubectl` and `helm` CLI tools.
+You can consider this group as a deployer's machine.
+
+The `k8s_control_plane` is where we deploy the K8s control plane.
+
+The `k8s_cluster` group must include all the K8s nodes including control plane
+and worker nodes.
+
+In case of running tests on a single-node environment the group `k8s_nodes`
+must be empty. This means the K8s cluster will consist of a single control plane
+node where all the workloads will be running.
 
 See for example:
 
@@ -30,7 +44,7 @@ all:
           ansible_host: 10.10.10.12
         node-3:
           ansible_host: 10.10.10.13
-    k8s_control-plane:
+    k8s_control_plane:
       hosts:
         node-1:
           ansible_host: 10.10.10.11