Add CONTRIBUTING.md

[vitorjmoriya]

📚 Documentation Issue Report

The problem

This is a necessary document in order to start contributing to this repo that should contain Git practices used (atomic commits, rebase to update branches, etc) and also the Swift style guide to be used in the project

Sought-for solution

• Add in the Contributing with the info describing the practices in this repo
  • Git practices
  • Swift Style used