DOC: the description of activating virtual env with pip under windows is not correct

[gbaychev]

Problem description

Hello everybody,
yesterday I tried to setup pandas dev environment under Windows. I have python only 3.7.5 installed.

In doc/source/development/contributing.rst is described how I can setup a dev environment and one of the steps is creating a virtual environment with pip. There is an instruction how to activate it:

. ~/virtualenvs/pandas-dev/bin/activate

This cannot function under Windows, since the virtualenv module does not create a bin folder. I've added a couple of comments which scripts should be run under Windows and will submit a PR shortly.