Add More Docs About Entities & Handlers

[salazarguille]

Add docs about:

• Entities
• Fields

in the schema file

[ControlCplusControlV]

@salazarguille mind if I take this one? I've been wanting to start contributing to yearn and can do technical writing

[salazarguille]

Hey @ControlCplusControlV yes for sure!

The idea is to add docs here in the repo using md files to help new contributors to understand the repo. Maybe creating a docs folder.

Thank you very much for your interest!!