docs: document Playwright test setup

Author: indradhanush

README.md

@@ -3,6 +3,18 @@
 
 Tracking days spent outside the UK. Free to use. Yours to keep. Forever.
 
+## Development
+
+Use Node 20 or later. Install the test dependencies and Chromium once:
+
+```sh
+npm ci
+npx playwright install chromium
+```
+
+Run `make test` to compile the TypeScript, run the model tests with coverage,
+and exercise the rendered application in Chromium with Playwright.
+
 ## Data retention, or the lack of it
 
 Everything you enter stays in your browser. Trips are saved to your browser's