Add contributing.json (GitMagic)

[ghost]

Thanks for trying out GitMagic.

Use this pull request to discuss and define what contribution guidelines you want to enforce in this repository. Here's a list of all available rules.

If you're happy with the set of rules as it is, you can go ahead and merge it.

If there is any rule that you are missing, or if you have any questions, send us an email at hello@gitmagic.io, we're happy to help.

Good luck!

How to test this PR

It is not possible to test this pull request without merging it. Once merged, you can verify that your rules are being enforced by either opening a new pull request or creating an issue.

_{This pull request was made by GitMagic – Magically enforcing your contribution guidelines.}

[coveralls]

Coverage remained the same at 89.796% when pulling ec0dca9 on gitmagic-bot:master into 503edd2 on pennsignals:master.