Add usage docs for playwright feature

src/playwright/NOTES.md

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+## Notes on usage
+
+This feature has been written to speed up the installation of [Playwright] within your devcontainers.
+
+Playwright is installed with `npm`, which means we need Node.js to be installed inside your devcontainer. You can achieve that by including these two features inside your `devcontainre.json` file:
+
+```json
+  "features": {
+			...
+    	"ghcr.io/devcontainers/features/node:1": {},
+    	"ghcr.io/gma/devcontainer-features/features/playwright:1": {},
+			...
+  },
+```
+
+While your devcontainer is building this feature will:
+
+- Temporarily install the latest version of the `playwright` package
+- Use Playwight to install any operating system dependencies required by the browers you want to use (i.e. it runs `npx playwright install-deps <browser>`)
+- Setup a named volume called `playwright-browsers` that Playwright will (later) use when you actually install some browsers
+
+By default the dependencies	for Chromium are installed. If you'd like to specify different browsers you can so by setting the "browsers" option:
+
+```json
+			...
+    	"ghcr.io/gma/devcontainer-features/features/playwright:1": {
+					"browsers": "firefox chrome"
+			},
+			...
+```
+
+Once the container is running, you'll take care of installing Playwright in your application as normal, with a command like this:
+
+```sh
+npm install --save-dev playwright@<version>
+```
+
+Once you've installed Playwright into your project, you'll be able to install the browsers you want to use:
+
+```sh
+npx playwright install chromium
+```
+
+The `playwright install` command will store the downloaded Chromium files in `/playwright/browsers`, which is mounted as a Docker volume. They'll therefore persist if you delete and then restart the container, or rebuild the image.
+
+[Playwright]: https://playwright.dev