chore(gatsby-plugin-guess-js): Update Readme on how to configure usage with CI

[Tian94-zz]

Adding documentation on how to configure the plugin to be used within a CI pipeline.

Description

Related Issues

[wardpeet]

Looks good! thanks for updating!

[wardpeet]

Seems like we have a flaky test. I'll be merging with my super powers.

Thank you for updating our docs! <3

[gatsbot]

Holy buckets, @Tian94 — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. We’ve got Gatsby t-shirts, stickers, hats, scrunchies, and much more. (You can also unlock even more free swag with 5 contributions — wink wink nudge nudge.) See gatsby.dev/swag for details.
2. We just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. Accept the invite by visiting https://github.com/orgs/gatsbyjs/invitation. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!