Enhance README with an example and instructions?

[schoettl]

Hi, I like morph and I think it's great to manage some servers/clients at work, including my own PC. Thank you for this work!

But to be honest, I didn't had an easy time to find out how it works and how I can use it. There are example files and a README but I had to search for some important options and had to discover how to best use morph.

Would you be open for a PR with a concrete example with explanation, like a guide for how to use morph for two simple hosts? That section could also contain some best practices on how to set the PermitRootLogin SSH option or what permissions are required and how to protect secrets in the repo.