give specific directions for running brandable_css
give specific directions for running brandable_css

flag=none

Test plan:
- Verify the directions in brandable_css error messages
  does build brandable_css

Change-Id: Ic34e05621b78c72842be91743c54f1cd3a79da18
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/299483
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jake Oeding <9601d161174d3602f0f7ee77da5115e75d23bb40@instructure.com>
QA-Review: Jake Oeding <9601d161174d3602f0f7ee77da5115e75d23bb40@instructure.com>
Product-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms