Add code for a Freedom Units demo app.

pyscriptjs/examples/toga/README.md

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+# Freedom Units!
+
+This is a demo Toga app implementing a Fahrenheit to Celsius converter.
+
+## Initial setup
+
+1. Create and activate a virtual environment:
+
+       $ python -m venv venv
+       $ . ./venv/bin/activate
+
+2. Install the demo requirements:
+
+       $ pip install -r requirements.txt
+
+### Development details
+
+This demo bakes a pre-compiled version of pyscript into the `server/pyscript`
+directory.
+
+It also includes an experimental version of toga-core, toga-web and toga-flask,
+packaged as wheels in the `server/wheels` directory.
+
+If any changes are made to the Toga sources or to PyScript, these vendored
+resources will need to be updated.
+
+## Web app
+
+The web app is a demo Flask server, serving a web app version of Toga at the
+root URL. To run the web demo server:
+
+    $ cd server
+    $ PYTHONPATH=../freedom/src python -m demo
+
+then point your browser at http://localhost:8081/
+
+Enter a value in the "farenheit" input, and click the "calculate" button.
+
+It may take a few seconds for this button to become live; look for the
+"Collecting nodes..." entry in the console log.
+
+## Desktop app
+
+To run this app in development mode:
+
+    $ briefcase dev
+
+To build and run an app bundle:
+
+    $ briefcase run
+
+If you're on an M1 macOS, this will raise an error on first run; if you get this error, run:
+
+    $ briefcase package -p app --adhoc-sign
+
+then re-run `briefcase run`
+
+## iOS app
+
+To run this in the iOS simulator, run:
+
+    $ briefcase run iOS
+
+To run this in the Android simulator, run:
+
+    $ briefcase run android
+
+Note that these builds have extensive requirements that must be installed -
+Xcode for iOS, and the Android SDKs for Android. These are multiple gigabyte
+downloads. Briefcase will detect when these tools aren't available, and either
+prompt you to download them, or perform a download for you. As a result, your
+first build may take up to 20 minutes to complete, depending on the speed of
+your conection.