Update README.md with more specific instructions around adding a pack…

…age (#1100)

README.md

@@ -63,7 +63,11 @@ In order to add a helm chart to this repository:
 [`partner-charts-ci` documentation](https://github.com/rancher/partner-charts-ci).
 `partner-charts-ci` can be obtained by running `scripts/pull-scripts`, which
 downloads the right version for your machine to `bin/partner-charts-ci`.
-3. Create a pull request for your changes targeting the `main-source` branch in this repository.
+3. Create a pull request for your changes targeting the `main-source` branch in
+this repository. **Please ensure that your pull request only contains changes
+related to the package that you are adding.** The `PACKAGE` environment variable
+is useful for this; for more information please see the
+[`partner-charts-ci` documentation](https://github.com/rancher/partner-charts-ci?tab=readme-ov-file#4-run-partner-charts-ci-update).
 
 
 ### Who is responsible for maintaining the helm charts in this repository?