[README] Add instructions for creating xcodeproj

[kaydeearts]

Description

Adding this so that we have clear instructions on how to create xcodeproj for debugging & development. Do we need more detail?

Checklist:

• [x] I have followed the Contribution Guidelines
• [ ] I have added test code and data to prove that my code functions correctly
• [ ] I have verified that new and existing tests pass locally with my changes
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation