Improve sign-posting for good first issues

[aleesteele]

Summary

In a discussion with @JimMadge at Collaboration Cafe, we were discussing how to make the repository friendly for beginners in the community. This might be easier for infrastructure or open source related folks, but might be more difficult for folks not familiar with these norms.

• Improving Contributing.md file: https://github.com/the-turing-way/the-turing-way/blob/main/CONTRIBUTING.md
• Improving README.md in the Github organisation
• Adding more tags to good-first-issue and good-first-pull-requests