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