Improve source documentation

[sethvincent]

Each dataset should have a readme (or description somewhere that's easy to find) with this info:

• source url
• publisher of dataset
• description of changes
• link to script for reproducing the geojson
• any relevant license information

[sethvincent]

Actually it would be nice if all the datasets had that info in a metadata object or something like that.

[stedy]

This group has something called datapackage.json which might suit our needs.

[sethvincent]

I just added a datapackage.json file! It seems pretty useful. I'll keep this open until I put all the types of metadata listed above into the datapackage.json file.