Update README.md (#2036)

Installing via yarn and npm

Author: Darkace01

README.md

@@ -29,10 +29,16 @@ This project is compatible with Expo/CRNA (without ejecting), and the examples h
 
 ## Installation
 
+Using NPM:
 ```
 $ npm install --save react-native-calendars
 ```
 
+Using Yarn:
+```
+$ yarn add react-native-calendars
+```
+
 The solution is implemented in JavaScript so no native module linking is required.
 
 ## Usage