Update contributing guide with steps to add an api endpoint

[bpvcode]

Describe what to do

• How to add an endpoint
• How our folders are structured
• How to add tests - Replace information from README.md to CONTRIBUTING.md
• How to add OpenAPI comments
• Change the commit messages section from README.md to CONTRIBUTING.md

Issue to resolve

#46

[bpvcode]

Hello @ageddesi

I am late on this, but already working on issue #46 as we talk :)
This is just a draft for now, but I am planning to have this done by the end of the day :)
Can you please assign this PR to me? Probably because I'm not a maintainer I can't assign an assignee or a reviewer.

Thank you :)

[bpvcode]

Hello @ageddesi ,

Everything is done now :)

Please review and if you need something else just tell me.

Thanks :)

[ageddesi]

Hi @bpvcode Perfecto, thank you for contributing 👍 Happy Hacktoberfest