fix(AndroidX support): Fixed broken import + added instructions (#583)

Hello, in order to use react-native-webview > 6.0.2 please make your android/gradle.properties contains:
```
android.useAndroidX=true
android.enableJetifier=true
```
This enables AndroidX libraries which are the new standard.

fixes #580 
fixes #581 
fixes #582

README.md

 
 Current Version: ![version](https://img.shields.io/npm/v/react-native-webview.svg)
 
+- [6.0.**2**](https://github.com/react-native-community/react-native-webview/releases/tag/v6.0.2) - Update to AndroidX. Make sure to enable it in your project's `android/gradle.properties`. See [Getting Started Guide](docs/Getting-Started.md).
 - [5.0.**1**](https://github.com/react-native-community/react-native-webview/releases/tag/v5.0.0) - Refactored the old postMessage implementation for communication from webview to native.
 - [4.0.0](https://github.com/react-native-community/react-native-webview/releases/tag/v4.0.0) - Added cache (enabled by default).
 - [3.0.0](https://github.com/react-native-community/react-native-webview/releases/tag/v3.0.0) - WKWebview: Add shared process pool so cookies and localStorage are shared across webviews in iOS (enabled by default).

android/gradle.properties

 ReactNativeWebView_kotlinVersion=1.3.11
 ReactNativeWebView_compileSdkVersion=28
 ReactNativeWebView_buildToolsVersion=28.0.3
-ReactNativeWebView_targetSdkVersion=28
-android.useAndroidX=true
+ReactNativeWebView_targetSdkVersion=28

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewManager.java

 import android.net.Uri;
 import android.os.Build;
 import android.os.Environment;
-import android.support.annotation.RequiresApi;
+import androidx.annotation.RequiresApi;
 import android.text.TextUtils;
 import android.view.Gravity;
 import android.view.View;

docs/Getting-Started.md

 
 React Native modules that include native Objective-C, Swift, Java, or Kotlin code have to be "linked" so that the compiler knows to include them in the app.
 
-This module does not require any extra step after running the link command 🎉
 ```
 $ react-native link react-native-webview
 ```
+
+iOS:
+This module does not require any extra step after running the link command 🎉
+
+Android:
+Please make sure AndroidX is enabled in your project by editting `android/gradle.properties` and adding 2 lines:
+
+```
+android.useAndroidX=true
+android.enableJetifier=true
+```
+
 For Android manual installation, please refer to [this article](https://engineering.brigad.co/demystifying-react-native-modules-linking-964399ec731b) where you can find detailed step on how to link any react-native project.
 
 For iOS, while you can manually link the old way using [react-native own tutorial](https://facebook.github.io/react-native/docs/linking-libraries-ios), we find it easier to use cocoapods.
   render() {
     return (
       <WebView
-        source={{uri: 'https://infinite.red'}}
-        style={{marginTop: 20}}
+        source={{ uri: 'https://infinite.red' }}
+        style={{ marginTop: 20 }}
       />
     );
   }