Add paragraph about signon data
Add paragraph about signon data

This commit adds a paragraph about signon data, to make clear that while it isn't synced from production, it is nevertheless available in the data dump.
alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet