update jumper docs with a reminder about xcode command line tools

Bug: skia:7633

Change-Id: I662cfb793924f03f65f2076bc150ca97525b1843
Reviewed-on: https://skia-review.googlesource.com/107780
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
1 file changed