I am using git version 2.7.0.windows.1 on a windows pc, I used the following command:
$ nano README
which results me:
bash: nan
If you already have nano install in your system, you just need to add the path of the exe file to PATH.
To do it just for Git bash, you can open GITINSTALL_DIR\etc\profile with any text editor and add this line to it:
export PATH="/DriveLetter/PATH/TO/YOUR/NANO:$PATH"
Of course you need to change the path above according to your case.
If you havn't installed it yet, just download and put it under GITINSTALL_DIR\bin or any folder included in $PATH