docs: mention templates in contributing section of README (#308)

Author: bcoe

packages/google-cloud-oslogin/README.md

@@ -109,6 +109,12 @@ More Information: [Google Cloud Platform Launch Stages][launch_stages]
 
 Contributions welcome! See the [Contributing Guide](https://github.com/googleapis/nodejs-os-login/blob/master/CONTRIBUTING.md).
 
+Please note that this `README.md`, the `samples/README.md`,
+and a variety of configuration files in this repository (including `.nycrc` and `tsconfig.json`)
+are generated from a central template. To edit one of these files, make an edit
+to its template in this
+[directory](https://github.com/googleapis/synthtool/tree/master/synthtool/gcp/templates/node_library).
+
 ## License
 
 Apache Version 2.0