[docs] Update documentation for branching process

Document new chrome_release_branch script.

No-Try: true
Docs-Preview: https://skia.org/?cl=209418
Bug: skia:8932
Change-Id: Ic38840f6e8ecac309e6bd7669dd2b110ff586401
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209418
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ravi Mistry <rmistry@google.com>
1 file changed