Docs improvement (pod installation)

[vanya2h]

First of all, thanks for awesome lib, but i want to suggest you to add some notes to your installation guides.

When I was trying to install react-native-in-app-utils via react-native cli, installer doesn't installed new pod dependency so I must do it by hands

I think it'll we helpful to other to add next step:

cd ./ios
pod install

[giangyger]

Absolutely! You just saved me a lot of time, thx :)