Add enableForegroundNotifications() to documentation

[besserwisser]

First of all: Thank you so much for this plugin, it helped a lot!

The README file does not mention the FirebasePlugin.enableForegroundNotifications() function. I have been trying to get the foreground notifications working on iOS for hours now until I found https://github.com/dpa99c/cordova-plugin-firebasex/issues/10 issue mentioning the enableForegroundNotifications function. After using it, the foreground notifications work (of course when using the data->notification_foreground attribute in the JSON). It would have helped a lot, if this necessary function call would be mentioned in the README.

Thanks

[dpa99c]

Issue #10 references PRs against cordova-plugin-firebase that would-be-nice to merge into this fork of the plugin.

Among them, Added API method enableForegroundNotifications() is a PR which disables foreground notifications by default and enables them via that function.

However that PR has not been merged into this plugin fork so enableForegroundNotifications() does not exist in cordova-plugin-firebasex.
That PR is unnecessary for this fork since display of foreground notifications can now be controlled entirely remotely using the notification_foreground key in the data message payload, hence I will remove it from #10.