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ARROW-1498: Add CONTRIBUTING.md to .github special directory

I don't think it's necessary to have an ISSUE_TEMPLATE.md right now, but we can add one in the future if it is deemed useful

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1176 from wesm/ARROW-1498 and squashes the following commits:

462e766e [Wes McKinney] Add CONTRIBUTING.md to .github special directory