Add yarn create example

[nojaf]

Changes

I added instructions to scaffold a new CSA using yarn.

Testing

Docs

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[melissamcewen]

I think npx create-snowpack-app new-dir --template [SELECT FROM BELOW] --use-yarn works if you want to use yarn? Does Yarn have its own npx?

[nojaf]

https://classic.yarnpkg.com/en/docs/cli/create/

[FredKSchott]

LGTM! Enough people prefer yarn that this makes sense.