Fix installation process

The `generate.js` file requires the `app/content/texts/` directory to exist. The new `.gitignore` and `.gitkeep` files will ensure the directory is there. The `.gitignore` file will make sure that no biblical texts in that directory get added to the project.

I also updated the `README.md` file to reflect the correct location of the `generate.js` file.

After I made these changes, I noticed that someone had made a [very similar pull request](https://github.com/digitalbiblesociety/browserbible-3/pull/3). I think the `.gitkeep` file is a more conventional way to include a directory without including its contents. It looks like Jay made another change to the `README.md` file that you will want to include in the project, though, even if you accept my pull request.