properties for exposing custom Braille descriptions

Hi everyone,

As tasked in a previous meeting, I've written up a very rough first draft
on the ARIA GitHub wiki at [1]. I've mainly copied and modified aria-label
and aria-role-description. The draft includes several questions at the end.

As I mentioned on the call, NVDA had done an experimental implementation of
such properties at a workshop in 2018 and Orca appears interested as well.

Thanks to Joanie and Sina for their help so far.

Best,
Peter.

[1]
https://github.com/w3c/aria/wiki/*-braille-properties-for-exposing-custom-Braille-descriptions