Add the creation of .env file in README.md

[persian-michael-scott]

Hi thanks for beautiful job. i started to clone your project but i got lots of errors.
here is some fixes :
1- should create the venv using python 3.11
2- for doing migrate step you should have a .env file in root project folder. it should contain :

db_username=postgres        #default user
db_password=                #the password that you give postgresql when installing
db_hostname=localhost        
db_port=5432                # default port
db_name=postgres            # default db name

secret_key=                 # a secure secret key with length 32
algorithm=HS256             # Commonly used algorithm for JWTs
access_token_expire_minutes=30   # Token expiration time in minutes