Add a concept of virtual environments

[ishant9805]

Sir, In your documentation, you have not mentioned any topic regarding virtual environment which is a must for the people who are learning python as it helps in package management, solves the version conflict problem, etc. Thus, benefitting the new learners to start working on any projects easily. Therefore, I would request you(@animator) to assign me this issue. I will try my best to solve it. With regards, Ishant