Add contribution guidelines to the project
Add contribution guidelines to the project

Since we're now heading for greatness thanks to Jessica we need to
get ourselves some contribution guidelines so that we can all work
well together without any conflicts (by explaining things like
tests and that we use the development branch etc.)

Also, we're a happy team! Woooo!

okfn/datapackage-py,okfn/datapackage-model-py,okfn/datapackage-py,okfn/datapackage-model-py,datapackages/datapackage-py,datapackages/datapackage-py,sirex/datapackage-py,sirex/datapackage-py