Add note about copying action setup from this project

Author: ianfixes

.github/workflows/README.md

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+## Note to `arduino_ci` users
+
+In this project, we define a workflow for each target platform.  **If you're looking for an example you can copy from, take only `linux.yaml`.**
+
+
+### Long version
+
+The reason that all platforms are tested in _this_ project is to ensure that, as a framework, `arduino_ci` will run properly on any developer's personal workstation (regardless of OS).
+
+For testing an individual Arduino library in the context of GitHub, [Linux is the cheapest option](https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-billing-and-payments-on-github/about-billing-for-github-actions) and produces results identical to the other OSes.