add install steps in macOS

[prafaelo]

You don't need to have installed the Github Desktop to use git - It's your option and freedom.

So, in the "install" section, add the steps to install git, for example:

$ brew install git
$ git version

# // This line is necessary in case the system still shows the oldest git version
$ PATH=/usr/local/git/bin:$PATH 

# Github Desktop, see https://github.com/desktop/desktop
$ brew install --cask github

Also, in section "install" shows "Github Desktop" as one GUI recommendation.