docs: add Contributing.md

[miklb]

Description

This pull request adds a Contributing.md and improves installation documentation. It also adds sections for commit messages, pull requests and review.

Addresses issue #6

List of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] documentation

Checklist:

• [x] This pull request relates to a ticket.
• [ ] This code is tested.
• [x] I want my code added to the response theme.