add a CONTRIBUTING.md file

add a CONTRIBUTING.md file, copied verbatim from the "Pull Requests" section in README.md

CONTRIBUTING.md

+Since this repo includes a large and diverse number of programming languages, frameworks, editors, and ecosystems, it's *very helpful* if you can provide a link to information supporting your pull request. Up-to-date, canonical documentation that mentions the files to be ignored is best.
+
+This ensures we can efficiently go through pull requests and keep quality high.