Mark promise progress / notify() as deprecated for Promise v2

[clue]

The promise progress / notify() API is deprecated and has been removed with the upcoming Promise v3 release (see #32). There's currently no schedule as to when this version is going to be released.

The current documentation for Promise v2 should include a deprecation notice to discourage people from relying on this API.

[slash3b]

@clue could you please check #109 ?

[clue]

Closed via #112