SkQP: Stop requiring users to use posix shell

Also update README.md

Change-Id: I62f5ac38ff4a8c3aa19d895441a76664cb8e8176
Reviewed-on: https://skia-review.googlesource.com/99302
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
9 files changed