Added README.md : Preview Video

Author: avinashbest

README.md

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+### Calculator App
+- Created a calculator which can perform all arithmetic operations like addition, subtraction, multiplication, division etc. depending upon the user input.
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+- The apk created can be used to install the application on different devices. 
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+- The layout of the Calculator is using TableLayout
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+- For calculation of the expression, the app is using ![exp4j](https://www.objecthunter.net/exp4j/) library (Expression for Java)
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+![preview](preview/preview.gif)