Browsers: Document user-gesture requirement for entering VR (Development FAQ)

[cvan]

In Chromium (and Firefox) folks have been confused about (or unaware of) the user-gesture requirement for entering VR mode.

Here's an example of the JS error that appears in the Console in Chrome for Android:

DOMException: Presentation request was denied. {code: 0, name: "NotAllowedError", message: "Presentation request was denied."}