Update instructions for running against rummager

The host that people using VMs will usually want to use now is
search.dev.gov.uk, so say that in the instructions.  Also, explain how
to get a search index replica with a single command, since most people
can use that rather than having to build their own.
Merge pull request #1643 from alphagov/update-readme-about-search

Update instructions for running against rummager