add tip for running browser tests in CONTRIBUTING.md

Author: coyotte508

CONTRIBUTING.md

@@ -22,6 +22,8 @@ If you want to run only specific tests, you can do `pnpm test -- -t "test name"`
 
 You can also do `npx vitest ./packages/hub/src/utils/XetBlob.spec.ts` to run a specific test file.
 
+Or `cd packages/hub && npx vitest --browser.name=chrome --browser.headless --config vitest-browser.config.mts ./src/utils/XetBlob.spec.ts` to run browser tests on a specific file
+
 ## Adding a package
 
 - Add the package name in [`pnpm-workspace.yaml`](pnpm-workspace.yaml)