Rename file with Git

I would like to rename file from README to README.md. What is the best practice to do that?

I have only one repo called "change-z-index".

1. I open and login like that:

   ssh -T git@github.com

And I enter my passphrase.

2. I try to rename the file like that:

   git mv README README.mdgit commit -m "renamed"git push origin master

It gives me an error saying bad source.

I think I need to select my repo first... its name is "change-z-index". I have read manual many times, but still can't understand how to do it.