experimental/documentation/gerrit.md: more detail

NOTRY=true
Change-Id: I386013af35ae2a8b51fa78109ebd89a826334839
Reviewed-on: https://skia-review.googlesource.com/7754
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
2 files changed