Create CONTRIBUTING.md

Create a CONTRIBUTING.md for helping contributors

## Expected Behavior

README should give just run instructions and CONTRIBUTING should give instructions to introduce changes and to check whether the changes work

## Current Behavior

There is a single README

## Possible Solution

## Context

It's difficult for contributors to find their way around unless they go through the whole code base.

## Your Environment

* Version used:
* Environment name and version (e.g. Chrome 39, node.js 5.4):
* Operating System and version (desktop or mobile):
* Link to your project:

Read the existing README first and compare its run instructions with the requested contributor guidance. Create CONTRIBUTING.md with instructions for introducing changes and checking that they work, while keeping the README focused on running the application; done means both documents have these distinct purposes.

Written by the indexing model from the issue text.