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
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