Create //playbooks/shell.md

Taken from the overview:
> I'm making this as an offline reference for some of the commands that I use
> often enough to need to remember but not often enough to *actually* remember.

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+# Shell
+
+I'm making this as an offline reference for some of the commands that I use
+often enough to need to remember but not often enough to *actually* remember.
+
+## Reference
+
+- To kill a process by its port number:
+
+```shell
+$ fuser 8080/tcp
+```