Added Contributing Guidelines

CONTRIBUTING.md

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+# How to contribute
+- Thank them for wanting to contribute
+- Explain there are a few guidelines to contribute
+
+Thanks for your interest in contributing to the AmplitudeJS project! We have a few guidelines on getting started and how to contribute:
+
+## Coding Guidelines
+
+- Be friendly and respectful of other people's ideas
+- Make sure all code is properly indented with tabs and commented thoroughly
+- Everything MUST be in Vanilla JavaScript and not revolve around any library
+- Spend some quality time testing your code
+
+## Set Up Your Development Environment
+1. Clone Github repository to your machine. Use the `dev` branch to access the latest and greatest code.
+2. All source code is located in the `src` folder
+
+## Creating a new build of `amplitude.js`
+1. Change directories into where you cloned the repository and run `npm install`
+2. Make your changes to the appropriate files in the `src` directory
+3. Run `npm run build` to process a build of your source code