Add note to help new users install

pip's installation guide can be a bit confusing for new Python users. As [demonstrated in this reddit post](http://www.reddit.com/r/Python/comments/1ca8g0/the_suggested_way_to_install_pip_is_with/), the installation guide seems to suggest that you should use virtualenv to install pip. The virtualenv installation guide says you should use pip to install virtualenv. While the relationship between the two might be obvious to experienced users, new users would easily get caught in this self-referential loop.

Adding this note might help new users get started with pip.