Switch from using rnpm to react-native cli in setup guide

react-native CLI is the modern way to go and should be encouraged.

README.md

 ## General Usage
 ```
 npm install --save react-native-permissions
-rnpm link
+react-native link
 ```
 
 Add permissions to manifest for android and info.plist for ios (xcode >=8). See notes below for more details.
 
 ````
 npm install --save react-native-permissions
-rnpm link
+react-native link
 ````
 
 ### Or manualy linking