Add some documentation about `mods=foo` syntax
Add some documentation about `mods=foo` syntax

Submitted for partial temperance of the entirely-justified wrath of
@philandstuff.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet