Add a README for ./docs

[ci skip]

Author: michaelklishin

docs/README.md

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+# Manual Pages and Documentation Extras
+
+This directory contains [CLI tool](https://rabbitmq.com/cli.html) man page sources as well as a few documentation extras:
+
+ * An [annotated rabbitmq.conf example](./rabbitmq.conf.example) in the [new style configuration format](https://rabbitmq.com/configure.html)
+ * An [annotated advanced.config example](./advanced.config.example) to accompany `rabbitmq.conf.example`
+ * An [annotated rabbitmq.config example](./rabbitmq.config.example) in the classic configuration format
+ * A [systemd unit file example](./rabbitmq-server.service.example)
+
+Please [see rabbitmq.com](https://rabbitmq.com/documentation.html) for documentation guides.