Docs/update configuration with custom fields (#NBLR-528)

[pedro-vasconcellos-vtex]

What problem is this solving?

• Add customFields to configuration.json example
• Update README style
• Minor fixes on README

How should this be manually tested? N/A

Screenshots or example usage: N/A

[vtex-io-ci-cd[bot]]

Hi! I'm VTEX IO CI/CD Bot and I'll be helping you to publish your app! 🤖

Please select which version do you want to release:

• [ ] Patch (backwards-compatible bug fixes)

• [ ] Minor (backwards-compatible functionality)

• [ ] Major (incompatible API changes)

And then you just need to merge your PR when you are ready! There is no need to create a release commit/tag.

• [x] No thanks, I would rather do it manually 😞

[vtex-io-docs-bot[bot]]

Beep boop :robot:

Thank you so much for keeping our documentation up-to-date :heart: