Move CONTRIBUTING.md and add badges

[eliotwrobson]

See title, more changes in prep for the upcoming review. Normally wouldn't care too much about this file placement, but this is specifically what is asked fro in their guidelines: https://www.pyopensci.org/python-package-guide/documentation/repository-files/contributing-file.html

Also added badges that are missing from the reviewer checklist: https://www.pyopensci.org/software-peer-review/how-to/reviewer-guide.html#peer-review-template

[coveralls]

coverage: 99.64%. remained the same
when pulling 186ecd5 on eliotwrobson:contributing_move
into 060f58c on caleb531:develop.

[caleb531]

Yep that's fine! I have no problems with moving CONTRIBUTING.md to the project root, and think it's actually more discoverable that way, anyway.