Adds not one but two additional 'notes' to the pip install instructions including a warning not to use sudo unless you know what you're doing, and a link to instructions for using virtualenv.  Also added --no-deps to the instructions by default, which I think will get fewer people into trouble.
Replace link to Stack Overflow answer with a link to the pip documentation--not much better more immediately relevant.
Merge pull request #798 from iguananaut/issue-798

add brief discussion of sudo to install docs