Add Slack link
Add Slack link

In the Prototype Kit documentation there's a reference to Slack if people get stuck during installation. But we don't include a link to the #prototype-kit Slack channel. It'd be good to include this.
dwpdigitaltech/hrt-prototype,alphagov/govuk_prototype_kit,alphagov/govuk_prototype_kit,dwpdigitaltech/hrt-prototype,dwpdigitaltech/hrt-prototype,alphagov/govuk_prototype_kit