Add link for parks donation to readme

[katiejolly]

I love these posters and national parks, too! This is such a great idea for color palettes. Just a thought - maybe include the link to donate to national parks on your readme? https://t.co/bZP7ha79Ip It would be so cool if your beautiful palettes helped the parks somehow

— maggi brisbin (@MargaretBrisbin) October 23, 2019

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