canvaskit: add helpful message to compile.sh

Change-Id: I48a48c8d2f3853fd948cadfc7cc93f41d72e2ea2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248800
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed