README: add more info about git workflow and not to open PR

[UniquePixels]

Because

Adds a bit more verbiage to encourage learners to use git workflow for these exercises but not to submit a PR.

This PR

• Add paragraph to intro of readme

Issue

Closes #465

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. 01-flex-center: Update self check
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [ ] If applicable, I have ensured that the TOP solution files match the Desired Outcome image