Add documentation of coding standard

[iyehuda]

What would you like to be added

We should extend the contribution guide (CONTRIBUTING.md) with documentation about coding standards in this project. We can alternatively create a HACKING.md file and put it there.

The documentation should include information about:

• Linting:
  • imports
  • literals
  • linting tools used in this project (CI/precommit hook)
• Testing:
  • Unit test standards
  • Integration tests (we should start making those)
• Architecture:
  • Data pipeline (event system)
  • Code location
  • Principles to add new code

Why is this needed

• Make code reviews shorter and save commenting about such topics
• Welcome new developers
• Preserve and share knowledge

[avats-dev]

@iYehuda I can work on this. Should I start? Also, I might need some help along the way. Thanks.