Add info about configuration location to docs

[cloo]

As a courtesy for people like me, who have not encountered .env file culture before, I ask to make a small improvement to docs.
It took quite an effort for me to find exactly where can I edit variables (especially when using Yunohost which does not explicitly declare sometimes multiple install locations). So please add to docs something like:
"Environment variables are stored in .env file in the main installation directory".