[Documentation] missing requirement setuptools

* added setupTools to (defined) Requirements section

README.md

@@ -5,8 +5,9 @@ It is smart enough to handle multiple remotes, branches, dirty working
 directories, and more, hopefully providing a great way to get everything
 up-to-date for short periods of internet access between long periods of none.
 
+# Requirements
 gitup should work on OS X, Linux, and Windows. You should have the latest
-version of git and either Python 2.7 or Python 3 installed.
+version of git and either Python 2.7 or Python 3 installed with the [setuptools](https://github.com/pypa/setuptools) extension (if it's missing, check for `pip install setuptools`).
 
 # Installation