Update Setup.md

[smorad]

Talked a user through setting up atc. Turns out that pip will silently fail to install the atcd binary to /usr/local/bin if they don't have write permission. Add sudo to the guide so they aren't as confused.

[chantra]

originally, the documentation mentioned using a virtualenv, which should install the files where the user has permission. Was the virtualenv step missing?

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[stasfilin]

@smorad you don't need sudo access for this, try to use virtualenv or pyenv