chore(docs): npm install instructions to docs (#616 by @a-c-sreedhar-reddy)

[skip ci]

docs/Getting-Started.md

 
 ```
 $ yarn add react-native-webview
+```
+ (or)
+ 
+ For npm use
+```
+$ npm install --save react-native-webview
 ```
 
 #### 2. Link native dependencies