Add BrowserStack mention to project ReadMe (#22207)

### Description
Condition for [BrowserStack support for open-source
projects](https://www.browserstack.com/open-source)

### Motivation and Context
- Considering using BrowserStack for our end-to-end tests for iOS and
Android

README.md

@@ -32,6 +32,8 @@
 |Web|[![Build Status](https://dev.azure.com/onnxruntime/onnxruntime/_apis/build/status/ONNX%20Runtime%20Web%20CI%20Pipeline?label=Web)](https://dev.azure.com/onnxruntime/onnxruntime/_build/latest?definitionId=161)||
 |Other|[![Build Status](https://dev.azure.com/onnxruntime/onnxruntime/_apis/build/status/onnxruntime-binary-size-checks-ci-pipeline?repoName=microsoft%2Fonnxruntime&label=Binary+Size+Check)](https://dev.azure.com/onnxruntime/onnxruntime/_build/latest?definitionId=187&repoName=microsoft%2Fonnxruntime)||
 
+This project is tested with [BrowserStack](https://www.browserstack.com/home).
+
 ## Third-party Pipeline Status
 
 |System|Inference|Training|