allow specifying #! inside shell

fixes #51

README.md

@@ -447,8 +447,19 @@ script can be overriden by running the script with
 
 ## Nushell scripts
 
-By default, the script contents are executed as a `bash` script, you can instead
-execute the script as a `nushell` script by using `.nu` as the file extention.
+By default, the script contents are executed as either `bash` script, or `nu` script based on the extension.
+
+You can overwrite the interpreter used to execute the script by adding an extra `#!` line to the script, for example:
+
+```bash
+#! /usr/bin/env -S haze script
+#! haze shell pgsql s3
+#! php -f
+<?php
+print("Hello");
+```
+
+Note that the interpreter used needs to already be available inside the haze container.
 
 ## Configuration