[#56] Update contributing doc

Description

Problem: We provide both stack/hpack(stack.yaml) and cabal(cabal.project and cabal.project.freeze) files in the repo. This redundancy can be confusing.

Solution: Describe cabal and stack files relationship in contributing doc.

Related issue(s)

Resolves #56 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/
  • I updated the changelog if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.