add command to install virtualenv package

[amr-atif]

the documentation is not clear on this point as we need to add a command to install virtualenv package to be able to create a virtual environment

$ python -m pip install virtualenv

and added a description before it. closes #133

[wlav]

I think that at this point (with most Pythons in the wild newer than 3.3), it's better to change the text wholesale to use venv with a reference to virtualenv just for old Pythons, rather than the opposite as is the case now.

[amr-atif]

I have edited the creation of venv commands so we can use venv instead of virtualenv as most people use modern versions of python. then I updated the description and added a note to explain how to use virtualenv for old versions of python.

[amr-atif]

What do you think about it this way, do you recommend a better way that will make it more clear? @wlav

[wlav]

Thanks!