DOC: Add a more welcoming tone for new contributors#17580

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TomAugspurger merged 4 commits intopandas-dev:masterfrom

willingc:docs-welcome

Sep 20, 2017

Commits on Sep 18, 2017

Commits on Sep 19, 2017

Edit per @gfyoung review
willingc
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Commits on Sep 20, 2017

Edit first 2 sentences and revert whatsnew change
willingc
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