SkQP: document requirement to run git-sync-deps

Change-Id: I78e77617b4f8a0c6c86d22cd9b4aceca36dcfb17
Reviewed-on: https://skia-review.googlesource.com/c/175587
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed