Draft text for guideline 2.6 Additional Sensor Inputs

A pull request for the draft text for guideline 2.6 "Additional sensor 
input" is now at

https://github.com/w3c/wcag21/pull/607

The suggested text is:

Guideline 2.6 Additional Sensor Inputs. Provide alternatives for user 
input via device sensors.

I am beginning to wonder whether we should really have 'Additional' in 
the Guideline title, or whether we could just call it "Device sensor 
input". The point is that input may not be additional if sensors are the 
only way for a type of input. While for some cases (like shake to undo), 
the guideline intends to make sensor input additional (by asking authors 
to provide alternatives), it also covers situations where there is no 
meaningful way to provide additional input (orientation) and the focus 
is on not interfering on the content level with device processing the 
sensor input.