update manual install doc in README.md

[sujameslin]

When you install react-native-firestack manually without using react-native link, you have to add

target '[project name]' do
end

to your Podfile that you created. Other wise you will get following error.

[!] The dependency `Firebase/Core` is not used in any concrete target.
The dependency `Firebase/Auth` is not used in any concrete target.
The dependency `Firebase/Storage` is not used in any concrete target.
The dependency `Firebase/Database` is not used in any concrete target.
The dependency `Firebase/RemoteConfig` is not used in any concrete target.
The dependency `Firebase/Messaging` is not used in any concrete target.

OS: MacOS Sierra
react-native: v0.40.0
Target: iOS 8.0