Update CHANGELOG.md
Update CHANGELOG.md

Add details for https://github.com/alphagov/govuk_frontend_toolkit/pull/186
- Add sass variables for discovery and live phases
alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit