migrate to dotenv for configs

[YashKumarVerma]

Fixes #74

Description

• lists all environment configs into a file named .env.example following the same convention that was used for the routes file.
• adds a line in the readme to inform users about .env.example usage

Motivation and Context

• allows hot swapping config sets.
• no need to pass values mentally.

How Has This Been Tested?

• locally, yes.
• no changes in test suite.

Screenshots (if appropriate):

Types of changes

**What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

**Go over all the following points, and put an x in all the boxes that apply.
**(If you're unsure about any of these, don't hesitate to ask. We're here to help!)

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[birm]

Looks reasonable to me :)