Some wrong commands in Windows

[NanaXiong00]

Describe the issue:
Because the package download command is missing in README.md, the following error occurs when running python -m virtualenv venv in Windows.

Suggestion:

Add command pip install virtualenv.

Describe the issue:
source runs on Linux. In Windows, we should run venv\Scripts\activate to activate a virtualenv.

Suggestion:
Update command to venv/Scripts/activate.

Describe the issue:
In Windows, we should use setx to edit environment variables instead of using export in this following places.

Suggestion:

Add the following description in the location shown above.

• Note - In Windows, we use setx command shown below:

setx FLASK_APP app.py
setx FLASK_ENV development
setx FLASK_DEBUG true

@john0isaac and @hemarina for notification.