chore(babel-preset-gatsby): Updated README to include babel presets

[oneknucklehead]

Description

If we don't pass reactRuntime and reactImportSource to babel-preset-gatsby in .babelrc.json, JSX is invalid in gatsby-browser.js. In order to validate these errors, I have added the presets according to the docs.

Related Issues

Fixes #34518

[imjoshin]

Hey @oneknucklehead ! Thank you for the contribution. 😄

Per @LekoArts 's suggestion, could you add the change of the second bullet point here as well?

So if I understand it correctly, two things would need to happen:

• Update https://github.com/gatsbyjs/gatsby/blob/master/packages/babel-preset-gatsby/README.md to include both settings
• Update https://www.gatsbyjs.com/docs/reference/config-files/gatsby-config/#jsxruntime to link to https://github.com/gatsbyjs/gatsby/blob/master/packages/babel-preset-gatsby/README.md (when it is used)

[oneknucklehead]

Sure! but can you help me out? I can't figure out which gatsby-config file to make the changes to.

[oneknucklehead]

@j0sh77 can you help me out?

[LekoArts]

@oneknucklehead Here's the file: https://github.com/gatsbyjs/gatsby/blob/master/docs/docs/reference/config-files/gatsby-config.md

[oneknucklehead]

Hey @j0sh77 can you please review the changes?