Added notes about k8s node draining to the knowledge base

knowledge base/drain a k8s cluster node.md

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+# Drain a K8S cluster node
+
+1. mark the node as unschedulable (_cordon_):
+
+   ```sh
+   $ kubectl cordon kworker-rj2
+   node/kworker-rj2 cordoned
+   ```
+
+1. remove pods running on the node:
+
+   ```sh
+   $ kubectl drain kworker-rj2 --grace-period=300 --ignore-daemonsets=true
+   node/kworker-rj2 already cordoned
+   WARNING: ignoring DaemonSet-managed Pods: kube-system/calico-node-fl8dl, kube-system/kube-proxy-95vdf
+   evicting pod default/my-dep-557548758d-d2pmd
+   pod/my-dep-557548758d-d2pmd evicted
+   node/kworker-rj2 evicted
+   ```
+
+1. do to the node what you need to do
+1. make the node available again:
+
+   ```sh
+   $ kubectl uncordon kworker-rj2
+   node/kworker-rj2 uncordoned
+   ```
+
+## Sources
+
+- [How to drain a node in Kubernetes]
+
+[how to drain a node in kubernetes]: https://linuxhandbook.com/kubectl-drain-node/