README.md: Add instruction to fork before cloning

[mao-sz]

Because

For consistency with other exercise repos, and to ensure learners are allowed more Git practice by pushing to their forks.

This PR

• Adds instructions to fork repo prior to cloning

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes changes to exercises, they are also changed in the solutions folder.