Add a 'developing' section to the readme

internal_docs/developing-locally.md

@@ -0,0 +1,9 @@
+# Developing locally
+
+You can test local changes to the prototype kit package by running `npm run start:dev`. This will create a new prototype from your local prototype kit package in your device's `tmp` directory and automatically run `npm start`.
+
+If you need more control over a local prototype generated by your local prototype kit package, run the following in an empty directory on your device. Make sure `npx` is running against the path to your local prototype kit.
+
+```bash
+npx ../location-of-your-local-prototype-kit-package create --version local
+```