add a guide to contributing to SkJumper

Change-Id: Icd40cf5eff3d2156a3ca00d7950059d5b77f48bf
Reviewed-on: https://skia-review.googlesource.com/15890
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed