add documentation

diagram for pathops change

TBR=caryclark@google.com
Bug: skia:8049
Change-Id: I2bbd04f1efad53e1197a45baf3ce98f0eb550512
Reviewed-on: https://skia-review.googlesource.com/132664
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
1 file changed