Explain how man pages are generated

Per discussion with @lukebakken.

Author: michaelklishin

docs/README.md

@@ -8,3 +8,20 @@ This directory contains [CLI tool](https://rabbitmq.com/cli.html) man page sourc
  * A [systemd unit file example](./rabbitmq-server.service.example)
 
 Please [see rabbitmq.com](https://rabbitmq.com/documentation.html) for documentation guides.
+
+## man Pages
+
+### Source Files
+
+This directory contains man pages that are are converted to HTML using `mandoc`:
+
+    gmake web-manpages
+
+The result is then copied to the [website repository](https://github.com/rabbitmq/rabbitmq-website/tree/live/site/man)
+
+### Contributions
+
+Since deployed man pages are generated, it is important to keep them in sync with the source.
+Accepting community contributions — which will always come as website pull requests —
+is fine but the person who merges them is responsible for backporting all changes
+to the source pages in this repo.