Gatsby: [docs] Explain why we use Yarn in the contributing docs

[jlengstorf]

We use Yarn for contributing to the Gatsby repo. We should write an explanation of why in "How to Contribute".

[sharynneazhar]

I can take a stab at this! 🙂

Is there anything specific you want to add besides an explanation of Yarn is?

[pieh]

We use yarn workspaces feature that comes really handy for monorepos:
https://github.com/gatsbyjs/gatsby/blob/99e237ad3841872d8f879b6bba3ce27bf259b51f/package.json#L80-L82

[mikelax]

There are a few sections in the tutorial part 0, from this section down that discuss getting set up with npm. This should be updated to use yarn, or at least prefer to use yarn and possibly give npm alternative.

[DSchau]

@mikelax I think we want that to remain that way! This issue is rather with the "Contributing" section, in that we recommend yarn from a contributing back to Gatsby perspective.