documenting the custom schemes handling

I am not native english so I cant create a pr for this, but we need to documente the lastest changes in custom schemes handling (https://github.com/apache/cordova-plugin-inappbrowser/pull/263 and https://github.com/apache/cordova-plugin-inappbrowser/pull/274)

the things to document:
for example to allow whatsapp custom scheme and twitter
+ add new config.xml preference with the custom schemes you want to support:
```
`
```
+ add event listeners customscheme:
```
inAppBrowserRef.addEventListener('customscheme', function (event) {
//do whatever you want here like:
window.open(event.url, "_system");
});
```

Review pull requests 263 and 274 first to understand the latest custom-scheme behavior, then find the plugin's current configuration and InAppBrowser event documentation. Done means documenting the AllowedSchemes preference with the whatsapp and twitter example and showing how to listen for the customscheme event.

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