Add handleOpenURL to documentation

# Feature Request

## Motivation Behind Feature
I want to be able to open specific file types with iOS app (file association if you will).
This is already implemented basically, but not documented.
A documentation of how to achieve this would be amazing :-) (including what needs to be added to the plist file)

## Feature Description
Add more documentation to `handleOpenURL` function which is currently not documented.
I needed to reverse engineer the code to find out this is already implemented.

## Alternatives or Workarounds
None, google search got me to 3.4 cordova docs where it is half mentioned there.
I eventually found out about this by trial and error and reading the code...

Don't get me wrong, I think you are doing a great job with the cordova project, but this is a very nice addition that can be easily implemented using the `edit-config` for the -plist file and a simple js code: `window.handleOpenURL = () => { ... }`

Locate the existing documentation entry or reference for `handleOpenURL` and read the related iOS guidance, if present. Document how to receive the URL and configure file associations through the plist using `edit-config`; done means the supported setup and usage are clear without reverse-engineering the code.

Written by the indexing model from the issue text.