The Contributing instructions in the README give my-app as the sample
app name when running `npm run create-react-app my-app`. This just adds
those directories to the .gitignore so they don't accidentally get
committed.

Test Plan:

```
npm pack
node global-cli/index.js testing --scripts-version /Users/vjeux/random/react-getting-started/create-react-app-scripts-0.0.1.tgz
```

make sure it shows

```
Creating the app testing at /Users/vjeux/random/react-getting-started/testing
```