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Sync CONTRIBUTING.md text with engine.git (#8004)

* Sync CONTRIBUTING.md text with engine.git

Turns out we had very similar text in engine.git.  This patch makes the text the same by adopting the text in engine.git.

* Update CONTRIBUTING.md
A
Adam Barth committed
f68343eeec05bba7ff4efaf52d559248545dfd9a
Parent: 311e77a
Committed by GitHub <noreply@github.com> on 2/9/2017, 4:50:29 AM