Add shebang recipe interpolation docs

A recipe like the following is valid as far as I can tell:

```just
greet name:
#!/usr/bin/env sh
echo "Hello, {{name}}!"
```

However, the Shebang Recipes doc does not mention this so it's difficult for users to discern what's possible.

I would happily open a PR but I see that the README now says contributions are not accepted :/