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>

doc/README.android-fastboot

@@ -38,6 +38,10 @@ CONFIG_G_DNL_VENDOR_NUM
 CONFIG_G_DNL_PRODUCT_NUM
 CONFIG_G_DNL_MANUFACTURER
 
+NOTE: The CONFIG_G_DNL_VENDOR_NUM must be one of the numbers supported by
+the fastboot client. The list of vendor IDs supported can be found in the
+fastboot client source code (fastboot.c) mentioned above.
+
 The fastboot function is enabled by defining CONFIG_CMD_FASTBOOT and
 CONFIG_ANDROID_BOOT_IMAGE.