README.md and hello.js reference contentful.json

[jpalmieri]

Running npm run setup no longer saves space ID and API keys in contentful.json (it saves them in .env files).

However, the README and hello.js seem to be out of date and say that the configuration will be saved in contentful.json:

This project comes with a Contentful setup command npm run setup.

This command will ask you for a space ID, and access tokens for the Contentful Management and Delivery API and then import the needed content model into the space you define and write a config file (./.contentful.json).

https://github.com/contentful/starter-gatsby-blog/blob/23ddf494988d392553abd867a74629793eb14b15/bin/hello.js#L11-L13

Although contentful.json is still sourced, it's confusing to see .env files generated when you're expecting to instead see contentful.json file.