README: add snippet for ADC
README: add snippet for ADC

Fixes #95.

Change-Id: Idd2cde40ad328f3e01f62b53e5a8255452795736
Reviewed-on: https://code-review.googlesource.com/10050
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go