Admin (package.json tweaks and CONTRIBUTING file)

/cc @jasondavies @mbostock 

Trivial bike-shed susceptible stuff that shouldn't get in the way of the 1.1.1 merge in #58, but added to a separate branch just in case you'd rather arrange package.json differently.

I haven't run into many [CONTRIBUTING guidelines](https://github.com/blog/1184-contributing-guidelines) in the wild but given the requirement for the contributor agreement it seems useful.