AFDO-Bisect: Add user documentation

This CL adds a README containing some information about what the script
does, what the arguments are, and how to invoke it, for user benefit.

BUG=None
TEST=None

Change-Id: I909c75e35545c7049b030fde2d080f6466addabe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/1718885
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Tested-by: Emma Vukelj <emmavukelj@google.com>
1 file changed