docs(Documentation): Expo and Troubleshooting (#145)

* Add fix for keyboard dismiss leaving viewport shifted in iOS 12

* Readme - added note about Expo and added troubleshooting section

README.md

 - [x] Android
 - [ ] Windows 10 (coming soon)
 
+_Note: React Native WebView is not currently supported by Expo unless you "eject"._
+
 ## Versioning
 
 If you need the exact same WebView as the one from react-native, please use version **2.0.0**. Future versions will follow [semantic versioning](https://semver.org/).
 Import the `WebView` component from `react-native-webview` and use it like so:
 
 ```jsx
-import React, { Component } from 'react';
-import { StyleSheet, Text, View } from 'react-native';
-import { WebView } from 'react-native-webview';
+import React, { Component } from "react";
+import { StyleSheet, Text, View } from "react-native";
+import { WebView } from "react-native-webview";
 
 // ...
 class MyWebComponent extends Component {
   render() {
     return (
       <WebView
-        source={{ uri: 'https://infinite.red/react-native' }}
+        source={{ uri: "https://infinite.red/react-native" }}
         style={{ marginTop: 20 }}
-        onLoadProgress={e=>console.log(e.nativeEvent.progress)}
+        onLoadProgress={e => console.log(e.nativeEvent.progress)}
       />
     );
   }
 
 Simply install React Native WebView and then use it in place of the core WebView. Their APIs are currently identical, except that this package defaults `useWebKit={true}` unlike the built-in WebView.
 
+## Troubleshooting
+
+- If you're getting `Invariant Violation: Native component for "RNCWKWebView does not exist"` it likely means you forgot to run `react-native link` or there was some error with the linking process
+
 ### Contributor Notes
 
 - I've removed all PropTypes for now. Instead, we'll be using Flow types. TypeScript types will be added at a later date.