llvm_tool: update README of cherrypick_cl.py

Update its README now that we made changes to the script.

BUG=chromium:1057428

TEST=none.

Change-Id: I62e198892bc9fff2a0f930705f9451d46b50b31d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2212423
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Jian Cai <jiancai@google.com>
1 file changed