README updated with initial setup instructions

[JerecTharen]

I'm completely new to this project and am looking to get involved with this project/organization. To start off with, I found that this project has a pretty bare bones README document and thought I could update it with some basics.

Specifically it's important that the README states:

• Supported NodeJS versions
• Git clone instructions
• Dependency installation instructions
• Environment variable setup instructions
• Application startup instructions

I am very open to feedback, especially for the environment variable instructions I have stubbed out. The comments I made for the SESSION_SECRET and PASSWORDMONGO were made based on assumptions from taking a look at the server.js file and may not be accurate.