README: install instructions miss required delta dependency

The current README says:

```sh
git clone https://github.com/dlvhdr/diffnav.git
cd diffnav
go install .
```

  `go install .` succeeds, but running `git diff | diffnav` fails at runtime if delta is not installed and available in PATH.

  It would help to update the installation section to explicitly mention delta as a required runtime dependency and how to install it.
