Explain how to use git efficiently in PRs

[Guillemdb]

@VicenteAR Asked about avoiding merge conflicts in PRs, and that is a topic that deserves its own tutorial.

We need to add a section about using git efficiently to avoid duplicated commits and merge conflicts in Pull requests. The new section will cover the following cases:

• Avoiding and resolving merge conflicts
• Cascading PRs that depend on each other
• Updating changes pushed to master after the PR was opened.