How to ignore a file which is already committed?

Question

Previously, the following was my .gitignore file:

...
config/database.yml
.DS_Store

Later I created an app_config.yml

Answer 1

that doesn't help when you want to start ignoring a file that is committed and you want to keep it in the project... since git rm --cached would delete it from git repo...

I found this How do I stop Git from tracking any changes to a file from this commit forward? which is what I was looking for..

hopefully it's useful for other people as well...