examples: add README.md to playbook/.

examples/playbook/README.md

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+
+# playbooks Directory
+
+Although this lives under `examples/`, it is more or less an organically
+growing collection of regression tests used for development relating to user
+bug reports.
+
+This will be tidied up over time, meanwhile, the playbooks here are a useful
+demonstrator for what does and doesn't work.