write contribution guide

[cmdoret]

We should have clear guidelines for potential contributors. According to standard practice, we need to add:

• a CONTRIBUTING.md file describing what contributions are welcome
  • It should make it easier for users to interact with the repo (e.g. submitting issues)
• A development guide page in the docs with technical information for developers
  • It should make it easier for contributors (internal and external) to setup the development environment and contribute to the project.

Objective: Add clear development and contribution guides to docs

Requirements:

• [ ] Write Development guide
• [ ] Write `CONTRIBUTING.md
• [ ] Both pages are indexed in the docs
• [ ] CONTRIBUTING.md links to dev guide

Resources:

• What a CONTRIBUTING.md file should contain
• Why a development guide is important
• Mozilla science lab: How to build a CONTRIBUTING.md