add GitHub Codespaces to README.md

[lloydchang]

Describe the issue add GitHub Codespaces to README.md

To Reproduce Steps to reproduce the behavior:

1. Go to https://github.com/freeCodeCamp/classroom/blob/main/README.md

Expected behavior A paragraph about GitHub Codespaces in https://github.com/freeCodeCamp/classroom/blob/main/README.md

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Optional - GitHub Codespaces Environment

If you have used GitHub Codespaces in other projects, doing the same in freeCodeCamp Classroom should be straightforward.

• https://docs.github.com/en/codespaces/overview

• https://docs.github.com/en/codespaces/getting-started

• If you want a ready-made dev environment in your browser, make a fork of this repository.

• Afterward, set up your NextAuth-related environment variables in the .env file.

• Please follow the "Setup Instructions" in the terminal for more information.

Within freeCodeCamp Classroom, GitHub Codespaces is on par with Gitpod so that you can use either.

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Additional context Relates to #349 and #355