Update adding-a-new-package.md (#2255)

## Checklist

- [-] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [x] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [-] I have not broken the cheatsheet

docs/contributing/adding-a-new-package.md

@@ -8,6 +8,7 @@ Replace `foo` with your new package name in the instructions below.
 4. `code package.json` and update the `description` field
 5. `pnpm install`
 6. `pnpm -w fix:meta`
+7. If your package is not a library (ie it's an entrypoint), add a `"private": true,` line to your `package.json`, then run another `pnpm -w fix:meta` for good measure.
 
 For any packages that you need to depend on, you can run