Add shebang trick to documentation

README.md

@@ -180,3 +180,20 @@ You can generalize this using `jq`'s `env` and set the message as a source envir
 # with MESSAGE defined on the source
 sh -c "cat /dev/random | base32 | head -c8 | jq -cR '{id: ., title: env.MESSAGE}'"
 ```
+
+For more complicated logic, writing an update script in the data directory is usually necessary.
+These can be written in any language.
+For languages like Python where you might want dependencies, a useful trick is to embed the setup the setup in the update script as a polyglot with bash:
+
+```python
+#!/usr/bin/env -S sh -c 'exec "$0.venv/bin/python3" "$0" "$@"'
+''''nix build --impure --expr 'with (import (builtins.getFlake "nixpkgs") {}); pkgs.python3.withPackages (p: with p; [feedparser])' -o sourcename.py.venv
+exit
+'''
+
+import feedparser
+```
+
+Running `bash sourcename.py` from the data directory will ignore the shebang, run the `nix build` command to set up the venv, and exit.
+Running `./sourcename.py` will interpret the shebang and use `sourcename.py.venv/bin/python3` as the interpreter.
+You may have to adjust the `nix build` command to fit your environment.