CEC: Add README for VTS tests am: 5a1c92594c am: d678e1e24a am: ac131439bd am: d5c68842c9

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/1584903

Change-Id: Iae9a0451a892bc6b7a67f0661c28cf76b33c6a21