README: add instructions/steps on how to use this together with `google/clasp`

Hello,

I would love to see some instructions on how to get this generated file/folder structure skeleton to work well with [google/clasp](https://github.com/google/clasp)'s set up, specifically where to place its `.clasp.json`, `appsscript.json`, etc, relative to the `src` folder of typescript-library-starter, so that `clasp push` would work perfectly.

TIA, and good work!