Add a README

README.md

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+Calculator.js: a node.js Demonstration Project
+==============================================
+An example node.js project, including tests with mocha, that behaves like
+a pocket calculator.
+
+The project contains a simple node.js application that exposes REST APIs
+to perform arithmetic on integers, and provides a test suite with mocha
+and chai.  The `mocha-junit-reporters` package is included to provide XML
+output that can be presented in a continuous integration tool like
+[Azure DevOps](https://azure.com/devops).
+
+To build, simply:
+
+1. Runs `npm install` to install dependencies.
+2. Runs `npm test` to run Mocha and execute the unit tests.
+