Add "Docker Hub" callout to Registry image Quick Reference

registry/get-help.md

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+[the Docker Community Forums](https://forums.docker.com/), [the Docker Community Slack](https://dockr.ly/slack), or [Stack Overflow](https://stackoverflow.com/search?tab=newest&q=docker)
+
+-	**Need a free, secure hosted registry?** Try [Docker Hub](https://www.docker.com/pricing).