Christmas house-cleaning

Adding much-needed changelog and contributor guidelines, and possibly other fixes.

@parndt do you want to merge some of the changes from #294 into this PR? Anything else we should do here? I'm going to deal with versioning separately (in a subsequent version) since it will change the existing API. Focusing hear on clean-up stuff.

I guess I can cherry-pick the changelog and contributing guidelines into `3-0-stable` with minor changes.