Update contributing docs

[kylewayneluck]

Closes #6517

[gatsbybot]

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[gatsbot]

Holy buckets, @kylewayneluck — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (Currently we’ve got a couple t-shirts available, plus some socks that are really razzing our berries right now.)
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If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!

[KyleAMathews]

Thanks!