Add a link to client library howto to the README template

[tmatsuo]

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 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.

[bcoe]

I think setting the timeout on a single RPC call is a pretty common customization, and I often forget how to do it. Having a blurb along with an exmaple I think would be good, just a small snippet below the blurb.

[bcoe]

I would put this directly below.

Supported Node.js Versions

Perhaps in

Advanced configuration

[summer-ji-eng]

Modified the sentence a bit.

This library is built on top of google-gax as its transport layer. 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).

I suggest we could add in section:

Using the client library
...
[Insert here]

Samples