Improve README.md / Docs

[MykalMachon]

Summary

Improve the README.md file and the docs in general. This includes a few things, but here are a few general examples of what to do:

Todos

• Create basic instructions for each function
  • addItems() docs
  • modifyItems() docs
  • clearCart() docs
  • getCart() docs
• spellcheck and general onceover of existing README.md