[docs] Clarify that fresh clone requires you to create a .env file

[thegrumpyenby]

Where: Repo Readme

Why: Associated error: [Error: Middleware “strapi::session”: App keys are required](https://forum.strapi.io/t/error-middleware-strapi-session-app-keys-are-required/24948)

What: Solution is outlined here in the Strapi community forum: https://forum.strapi.io/t/error-middleware-strapi-session-app-keys-are-required/24948/1