Add Building a Github App

[kartikeytripathi]

Description

This tutorial is for building a Github App using Node.js.
This blog post describes the development of gifbot, a simple GitHub App, that responds to comments with animated GIFs:

Motivation and Context

It is a new tutorial. It will give a broad understanding of how GitHub app works.

How Has This Been Tested?

gifbot responds to webhook events which are raised when comments are added to a repo.

Types of changes

• Content Update (change which fixes an issue or updates an already existing submission)
• New Article (change which adds functionality)
• Documentation change

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have made checks to ensure URLs and other resources are valid