chore: add docs for experimental 'watch mode' (#33988)

CONTRIBUTING.md

@@ -26,6 +26,16 @@ npm run watch
 npx playwright install
 ```
 
+**Experimental dev mode with Hot Module Replacement for recorder/trace-viewer/UI Mode**
+
+```
+PW_HMR=1 npm run watch
+PW_HMR=1 npx playwright show-trace
+PW_HMR=1 npm run ctest -- --ui
+PW_HMR=1 npx playwright codegen
+PW_HMR=1 npx playwright show-report
+```
+
 Playwright is a multi-package repository that uses npm workspaces. For browser APIs, look at [`packages/playwright-core`](https://github.com/microsoft/playwright/blob/main/packages/playwright-core). For test runner, see [`packages/playwright`](https://github.com/microsoft/playwright/blob/main/packages/playwright).
 
 Note that some files are generated by the build, so the watch process might override your changes if done in the wrong file. For example, TypeScript types for the API are generated from the [`docs/src`](https://github.com/microsoft/playwright/blob/main/docs/src).