Refactor CONTRIBUTING

When testing out the current workflow in the CONTRIBUTING document I noticed something I think should be added:

I think we should add a section (maybe under Initial setup) on how to set the upstream for LFP's master branch on the developer's local clone of a repository. Also, maybe specify that this upstream is pointing to the original project in LFP.

This is used later when fetching changes from the upstream before merging but it was never defined how to set this up.