simple instructions for zsh

[super-brian]

As all of you know, now macbook comes with zsh, not bash.
But the instructions are not so clear for zsh.
I had to install Homebrew first, which took some time, but it was pretty good. (I had to enter computer password a few times)

1. But the under "Install Bash git completion", something had to be added to ".bash_profile" but I don't think it will work for zsh.
2. Then I am supposed to enter "source ~/git-prompt.sh" to .bash_profile, but I don't think zsh reads this either. So I put that into .zshrc, and it seems working, but not sure if this is safe or not.

So, I guess, it would be just nice to write a section for those who uses zsh, not bash.