Add windows virtualenv activate instruction

Closes #1075.

docs/tutorial/quickstart.md

@@ -12,7 +12,7 @@ Create a new Django project named `tutorial`, then start a new app called `quick
 
     # Create a virtualenv to isolate our package dependencies locally
     virtualenv env
-    source env/bin/activate
+    source env/bin/activate  # On Windows use `env\Scripts\activate`
 
     # Install Django and Django REST framework into the virtualenv
     pip install django