Update README instructions for using Wraith
Update README instructions for using Wraith

Add list of dependencies.

Add further information for baseline and feature branches.

gavboulton/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements