Kubernetes Hardening Guide added (#5718)

books/free-programming-books-subjects.md

@@ -140,6 +140,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Cloud Computing for Science and Engineering](https://cloud4scieng.org/chapters/) - Ian Foster, Dennis B. Gannon (:construction: *in process*)
 * [Cloud Design Patterns](https://docs.microsoft.com/en-us/azure/architecture/patterns/)
 * [Designing Distributed Systems](https://azure.microsoft.com/en-us/resources/designing-distributed-systems/) *(account required)*
+* [Kubernetes Hardening Guidance](https://media.defense.gov/2021/Aug/03/2002820425/-1/-1/1/CTR_KUBERNETESHARDENINGGUIDANCE.PDF) - NSA, CISA (PDF)
 * [Learn Azure in a Month of Lunches](https://azure.microsoft.com/mediahandler/files/resourcefiles/learn-azure-in-a-month-of-lunches/Learn_Azure_in_a_Month_of_Lunches.pdf) - Iain Foulds (PDF)
 * [Monitoring Modern Infrastructure](https://www.datadoghq.com/ebook/monitoring-modern-infrastructure/) *(account required)*
 * [Multi-tenant Applications for the Cloud, 3rd Edition](http://www.microsoft.com/en-us/download/details.aspx?id=29263)