Documentation on how to use popolo

Some example of how to model some common parliamentary structures to
give people a started on how the Popit data model works

For mysociety/popit#652
Initial version of the tutorial

Covers basics of adding people, organizations and memberships plus
updating people
Add brief page listing API client libraries

Just names, URLs and a mailing list CTA
improve modelling text and remove extraneous data

Expand/clarify the text.

Also, remove fields from the examples that aren't pertinent to make the
important fields more obvious.
correct error about how PUT API works