Add git to prerequisites

Resolves #3786

README.markdown

@@ -17,6 +17,7 @@ __Disclaimer:__ _Oh My Zsh works best on Mac OS X and Linux._
 * Unix-based operating system (Mac OS X or Linux)
 * [Zsh](http://www.zsh.org) should be installed (v4.3.9 or more recent). If not pre-installed (`zsh --version` to confirm), check the following instruction here: [Installing-ZSH](https://github.com/robbyrussell/oh-my-zsh/wiki/Installing-ZSH)
 * `curl` or `wget` should be installed
+* `git` should be installed
 
 ### Basic Installation