Add a 'Fog Credentials' section to README

Setting up the Fog credentials are not well documented in Fog, so
provide our users with clear guidance.
Merge pull request #24 from alphagov/credentials_in_readme

[#69702304] Add a 'Fog Credentials' section to README
Merge branch 'master' into 62223252_query_integration_test
:href was accidentally tested for, should be :name