Add content to add cluster page of docs

docs/clusters/add-cluster.md

@@ -0,0 +1,7 @@
+# Add Cluster
+
+Add clusters by clicking the Add Cluster button. You'll need to obtain a working kubeconfig for the cluster you want to add. You can either browse it from the file system or paste it as a text from the clipboard.
+
+Selected cluster contexts are added as a separate item in the left-side cluster menu to allow you to operate easily on multiple clusters and/or contexts.
+
+**NOTE**: Any manually added cluster is not merged into your kubeconfig file.