Added README.md file

Author: AnandD007

Calculator-GUI/README.md

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+<h1 align="center">Calculator (GUI)</h1>
+It is GUI based calculator used to calculate any simple mathematical equation.
+
+---------------------------------------------------------------------
+
+## Modules Used
+- tkinter
+## How it works
+- It takes the mathematical equation by the User.
+
+- It returns the result of the mathematical equation.
+
+#### By [Avinash Kr. Ranjan](https://github.com/avinashkranjan)