Add sample .env

[NicholasLYang]

Is your feature request related to a problem? Please describe.
I'm trying to deploy YACS and I don't have a reference for what environment variables the .env file should contain. I've tried grepping for uses of ENV in the Ruby code, but I'm not sure if that's all of the variables.

Describe the solution you'd like
Include a .env.sample that has dummy values. Or an .env.development that includes development values.

Describe alternatives you've considered
Or just put in the documentation what environment variables are necessary to run in prod.

Additional context
Nope. Thanks for all your work! YACS seems pretty awesome!