Add a link to `client library howto` to the README template

The README in our client libraries doesn't provide a document for how to customize client library's behavior (timeout, retry, etc). I suggest we add a link to the [Client Library Howto](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md) in the READMEs (if that document is not permanent, we can use a specific commit hash).

Here's a draft:

> This library is built on top of google-gax. To customize the common behavior of this client library (timeout, retry, etc), see [Client Library Howto](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md).