add gitignore file to git code example

Example 1: how to put files into gitignore

$ echo debug.log >> .gitignore
$ git rm --cached debug.log
rm 'debug.log'
$ git commit -m "Start ignoring debug.log"

Example 2: what to add to gitignore

Look at this awesome site!
https://www.toptal.com/developers/gitignore
Generates a gitignore for you, based on the technologies you're using

Example 3: how to git ignore a file

git config --global core.excludesfile ~/.gitignore_global