CONTRIBUTING.md getting too big

[mfisher87]

I find the doc a bit overwhelming to traverse to find the thing I'm looking for. I think it could stand to be made in to a category. Perhaps subpages would look like: contributing/development.md (e.g. "Here's how you set up your environment"), contributing/releasing.md (e.g. "Here's how we use semver"), contributing/maintenance.md (e.g. "Here's what maintainers do and how to be come one"), and contributing/explanations.md (e.g. "Here's why we use semver").

[mfisher87]

Resolved in #533