First pass at Contribution Guidelines

Started on some contribution guidelines
I always get the markdown link syntax backwards...
Applied review feedback.
Whoops! Updated "unscheduled" in one place, but not every place.
Added sample of code style and more notes on branches

(Please feel free to suggest more code style items!)
Merge pull request #852 from NuGet/anurse-AddContributionGuideline

Contribution Guidelines