update readme

Author: ad3n

README.md

@@ -2,9 +2,21 @@
 
 This calculator use [Strategy Pattern](http://designpatternsphp.readthedocs.io/en/latest/Behavioral/Strategy/README.html) to make it flexible, extendable, and robust.
 
+## How to install
+
 ## How to build
 
 ```bash
+composer require kejawenlab/pph21
+```
+
+or you can clone directly
+
+```bash
+git clone https://github.com/KejawenLab/PPH21.git
+
+cd PPH21
+
 composer update --prefer-dist -vvvv
 ```