[bazel] Update contribution docs
[bazel] Update contribution docs

Change-Id: Id9265dc8b3bc31d02755ea368b5e7b7fa944dfea
Bug: skia:12541
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/584219
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia