Add doc string for resource.register()

Hi,

I haven't seen any docs on this. I would like to add odo support to pandasdmx. It should be about 3 lines. But without documenting the register API even the simplest task turns into an uphill battle. In particular, what
are the kwargs good for? Can I pass args to the registered callable? By accident I found out that the kwargs contain a dshape item...

Start by locating the resource.register() implementation and reading how it handles kwargs and registered callables. Add a concise doc string covering the kwargs, including the dshape item, and whether arguments can be passed to the registered callable; done means these API questions are answered in the function documentation.

Written by the indexing model from the issue text.