Add section to README on indexing in development
Index the organisations field for an artefact

Modify the RummageableArtefact class to include an array of organisation 
ids for an artefact, which is consistent with the document structure 
of Whitehall documents.
Bump Rummageable to 1.0.1
Define a list of valid keys, now that Rummager doesn't
Add a SearchIndex class

This is a thin wrap around a Rummageable::Index instance.
Make RummageableArtefact work with Rummageable 1.0

This replaces the existing calls to Rummageable in RummageableArtefact
to now request an instance of SearchIndex, and then call the new-style
Rummageable methods on that.
Fix test helpers
Merge pull request #143 from alphagov/index-organisation-ids

Include organisations in the search index (and upgrade Rummageable)

https://www.pivotaltracker.com/story/show/67016644