more notes and notes in major directories

cmd/README.md

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+# `/cmd`
+
+Main applications for this project.
+
+The directory name for each application should match the name of the executable you want to have (e.g., `/cmd/myapp`).
+
+Don't put a lot of code in the application directory. If you think the code can be imported and used in other projects, then it should live in the `/pkg` directory. If the code is not reusable or if you don't want others to reuse it, put that code in the `/internal` directory. You'll be surprised what others will do, so be explicit about your intentions!
+
+It's common to have a small `main` function that imports and invokes the code from the `/internal` and `/pkg` directories and nothing else.