[sensors] Updating privacy considerations - accounting

lknik has just created a new issue for https://github.com/w3c/sensors:

== Updating privacy considerations - accounting ==
As per 
https://github.com/w3ctag/spec-reviews/issues/115#issuecomment-257227074

Let's add:

"High frequency readout can potentially be used for increasing 
monitoring of user's environment for unexpected purposes"

> "Implementations MUST enable the user to verify the current and past
 use of a sensor, together with requested frequencies."

It's for transparency/accounting. 

Please view or discuss this issue at 
https://github.com/w3c/sensors/issues/148 using your GitHub account