docs: document SDK demo prerequisites

README.md

@@ -56,6 +56,14 @@ import { MatrixActor } from '@open-matrix/sdk';
 
 ## Quick Start
 
+Prerequisites:
+
+- Node.js 22+
+- pnpm 10+
+- `nats-server` on `PATH` for the no-account standalone broker demo
+- Google Chrome/Chromium on `PATH`, or run `pnpm exec playwright install chromium`,
+  for the browser proof in `prove:fresh-samples`
+
 ```bash
 git clone git@github.com:matrix-sdk/matrix-sdk.git
 cd matrix-sdk