Finish main README

[StephenCathcart]

Description 🤔

Some parts of the main README is incomplete such as the API documentation section.

https://github.com/the-pragmatic-dev/meco-api/blob/master/README.md

Acceptance Criteria 🎉

• Written doc blocks, inline documentation or READMEs

Please read CONTRIBUTING.md for details on our code of conduct.