DOC: Update contributing guidelines

[Lucia-Fonseca]

Description

This PR updates the contributing guidelines, readme and documentation to explain how externals could contribute either with their own code or help us code some feature of our roadmap. This will partially address the feedback from the NumFocus application. Merging this PR closes #537

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[Lucia-Fonseca]

Are we happy to merge this?