Add comment explaining that using mirrors might break git cl.

No-Try: true
Docs-Preview: https://skia.org/?cl=220496
Change-Id: I987d912c0c62e407672d2b16631dc72a37749aab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220496
Reviewed-by: Heather Miller <hcm@google.com>
Auto-Submit: שחר לנגבהיים <nihohit@gmail.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed