Add contributing guidelines

[richfitz]

Motivated by seeing #213 appearing (which is very exciting) this seems like a good time to add contributing guidelines: https://github.com/blog/1184-contributing-guidelines

Basically, at a minimum, we need people to agree, in each PR, that they accept that the data will be released under the CC0 waiver. This can be added as a comment or statement in the PR and will mean that we have written permission for each request (Daniel already has these for the published compilation via email).

cc: @dfalster, @RemkoDuursma