Ignoring Files on a Local Repository

We maintain maintain many websites on the Pantheon Platform.  One of the things that we must do when creating local repositories is create a settings.local.php file for storing our local credentials so this information is not shared, nor does it break the site when you bring it down and push it back up.  I wish to not have git continually asking if I wish to track this file and I do not wish to take a chance of commiting it accidentally so, with help, I was told of this method to ignore the file locally and not have that addition to .gitignore not conflict or interfere elsewhere.