Upgrade yarn, update docs

[rygine]

in this PR:

• upgraded yarn to latest stable version
• updated README and CONTRIBUTING guidelines
• added .node-version and .nvmrc for use with node version managers
• updated LICENSE files

no release needed for these changes.

[changeset-bot[bot]]

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Latest commit: 9a272ebf48d31d943d82252944c81358cf0b241d

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