npm script: serve-dev: ignore BookReader folder when running serve-dev

While running development server. using `npm run serve-dev` it generates a folder in root project directory named `BookReader`

So whenever `git status` is executed in terminal while `serve-dev` is active or even ran once. It shows unnecessary files and folder to stage in git.

Expected: It should not show generated/compiled files in git stage
Solution: adding `./BookReader` in `.gitignore` might solve the issue