Update README for FCM

[alexsegura]

Hello,

When using FCM instead of GCM, some lines should be removed from AndroidManifest.xml, as explained here.

If you don't remove them, notifications are received twice 🙂

I updated the README.

If you want, I can rework the documentation a little more to explain usage with FCM, this would fix some issues as the library is working well with FCM.

[heyman333]

hello @alexsegura
if above lines were deleted, we won't be able to use onNotification method anymore

[alexsegura]

With this configuration it works, and onNotification is called.

Please note that I'm speaking about using FCM 🙂

[iamarkdev]

Our fork has FCM working.

https://github.com/Fanapptic/react-native-push-notification

[Gp2mv3]

Can we merge #717 which also includes changes done in this PR ?

[alexsegura]

@Gp2mv3 yes, you can close this one then. Thank you.