docs: Mention USB permissions in Getting Started
docs: Mention USB permissions in Getting Started

Adds a quick note to the "Running Tests on a Device" section that
a user may need to do some environment specific work to ensure
they have adequate USB device permissions before continuing.

Change-Id: I640b964646fde9178a9661cfc72eff7d911bdcbe
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13824
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed