Fix for Appstate changes causing disconnect

[ranjandsingh]

Closes #57

[EmilJunker]

Instead of implementing this in the library itself, creating a dependency on another package ('react-native'), adding yet another config option, and making it unnecessarily complicated to debug, I think it would be better to just add instructions to the readme how one can stop and restart the SSE connection when the app switches between background and foreground state.

I think something like this should do the trick:

import { useEffect } from 'react';
import { AppState } from 'react-native';

useEffect(() => {
    const appStateSubscription = AppState.addEventListener('change', (nextAppState) => {
        if (nextAppState === 'active') {
            // App became active, reconnect SSE
            es.open();
        } else if (nextAppState === 'background' || nextAppState === 'inactive') {
            // App went to background, close SSE connection
            es.close();
        }
    });

    return () => appStateSubscription.remove();
}, []);

[wojciechkrol]

@ranjandsingh @EmilJunker Thank you for pointing out this issue. I have added instructions to the README on how to address it, following @EmilJunker's recommendation. I will not merge this PR to avoid adding unnecessary dependencies.