Github workflow to contribute to existing repository

[jmoldon]

We need to add a subsection on how to contribute to repositories (fork-clone-push upstream). I like how the workflow is explained here, so we could follow the same approach:

https://github.com/spyder-ide/spyder/wiki/Dev%3A-Github-Workflow

Not everything is needed, but I want to include for example:

• The section Developers
• The workflow in Rebasing

There are other similar resources, but this is one option.