Fixed #16205 -- Added beginner guide for windows.

This is mostly the work from https://code.djangoproject.com/attachment/ticket/16205/16205.diff
I have removed "Setting up ENV vars" as decided as Users must know this, instead just written to ensure that ENV vars have been set up.
Removed "Common Pitfall" because the issue in that is because of PATH var only, I have faced this problem when installing Django in my friend's computer. And any other issue doesn't come to my mind which should be a 'common pitfall'