add .gitignore file to repo 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 is add.gitignore

The purpose of gitignore files is to ensure that certain files not tracked by git remain untracked.