added proper readme

README.md

@@ -1,3 +1,18 @@
 # OhThatsA [![Build Status](https://drone.broodjeaap.net/api/badges/broodjeaap/OhThatsA/status.svg)](https://drone.broodjeaap.net/broodjeaap/OhThatsA)
 
 An app to help you learn how to calculate day of the week from any date.
+
+## App
+<img src="media/img/main.png" alt="main" width="200"/>
+<img src="media/img/year_instruction.png" alt="year_instruction" width="200"/>
+<img src="media/img/month_practice.png" alt="month_practice" width="200"/>  
+
+Mostly made to try out Dart/Flutter.
+
+## Method
+
+The day of the week can be caculated by:  
+`(day value + month value + year value + century value + leap year value) % 7`
+
+Every one of these value comes from a set of constants or is calculable from the date.  
+This app helps you practice memorizing and calculating these values.