Update readme on how to connect to MongoDB free cluster

[Ajknight121]

We will use mongodb cloud to store data. Please create a section in the readme on what env variables need to be set for a MongoDB connection

Definition of Done:

• Using the dotenv library, use env variables to get connection to mongodb.
• create a .env file that declares the names of env variables but does not set them equal to anything
• use .env.local to keep a personal connection to mongodb
• Section in readme created about mongoDB