Document how to pass options to postNotification

[leoskyrocker]

I came across a few unresolved issues related to how to pass additional options to postNotification.
It is undocumented currently but actually implemented since v3.0.5 via #266.
It is now possible to call postNotification like this:

const otherParameters = {subtitle: 'subtitle', headings: {"en": "English Title"}}
OneSignal.postNotification(contents, data, playerId, otherParameters)

where otherParameters is anything options supported by create-notification endpoint.

Unresolved but relevant issues:
#262
#142
#253
#218

I'll try to see if I have time to submit a PR in the next few days.
For now, this can at least still serve as an information post for other people looking for the solution.

[leoskyrocker]

Closing as it got documented in 5b69508.

Thanks @Nightsd01!

[leoskyrocker]

@avishayil you could probably close all the other issues too =)