add: README file.

Author: rafaeldo

README.md

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+# React.js GIT COMPARE
+
+![Demo](https://i.imgur.com/VLRFCyw.png)
+
+Single Page App to compare Github repositories.
+
+## Getting Started
+
+1. Clone the repository.
+
+2. In root folder run `yarn install` or `npm install` to install all dependencies.
+
+3. In root folder run `yarn install` or `npm start` to run the App in dev mode.
+
+4. Visit `localhost:3000` to access the app in the browser.