Document Purchase

[jjeffryes]

Create the documentation needed for #5.

The documentation should be done in the Endor repo, so its publicly accessible, and should have all the data needed for a developer to create a purchase message that can be sent to a vendor's node and result in the vendor's node recording the purchase.

[jjeffryes]

Documentation tasks should be tracked in https://github.com/OpenBazaar/official_docs now.