Details on how to add new environment variables

[amitait]

I wanted to make it more clear for new devs who are not usually deal with environment variables and might tend to skip this step if they don't know how to use it.

[dylanshine]

The use of environment variables in the README is meant to be somewhat arbitrary, going to leave it up to devs as to how they would like to implement their variables based on whatever framework they're using.