Add better documentation on using DPM programmatically

[bnvk]

DPM itself bundles up datapacage-init and datapackage-read and other individual modules. If a developer wants to use these modules in an application is it advised to use dpm or rather those modules individually?

If the former is ideal, perhaps we should have better documentation on the README about this. If the later, perhaps noting that as well :-)

[bnvk]

@rgrp i'm happy to fork an add either to the docs as I'm working with all these modules individually at the moment!

[rufuspollock]

@bnvk my sense is that the "node" way is to use the lower level modules - though sometimes DPM may provide features you want and so you use that too.

Please dive in docs - pull requests will be rapidly merged :-)

[pwalsh]

@bnvk we'd welcome any input here :)