Document usage with Wave/Focus

[andybak]

I only knew there was support in VIU for the Focus because a mod on the Wave SDK forum told me.

There's nothing in the docs to indicate this and no info on how to handle the unique aspects of the Focus.

The docs could do with expanding in general.

It might even be time to consider a name change - calling this "Vive Input Utility" reminds me of the joke about the Holy Roman Empire - it was neither holy, roman nor an empire...