Adds a CLA to contributing.md#585

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mrennie wants to merge 2 commits intoCSSLint:masterfrom

mrennie:issue584

Commits on Apr 10, 2015

https://github.com/CSSLint/csslint/issues/584
mrennie
committed

Commits on Feb 1, 2016

https://github.com/CSSLint/csslint/issues/584
mrennie
committed