doc: README.android-fastboot: Add note about vendor ID

The Android fastboot client only communicates with specific vendor IDs.
This addition to the documentation points out that fact so everyone is
aware that not just any vendor ID will work and where to find the IDs
that will.

Signed-off-by: Clifton Barnes <cabarnes@indesign-llc.com>
Cc: Rob Herring <robh@kernel.org>