Update readme.
Update readme.

* Update instructions on how to create (and now migrate) indices
* Minor terminology corrections
* Add BSF and Trade Tariff to list of indexable apps
* Remove the time estimate, as it's out of date and varies depending
  on how old your database dump is
* Link to the instructions for Whitehall
alphagov/rummager,theodi/rummager,theodi/rummager,alphagov/rummager