Add text to README to join BrowserStack open source program

[Thahara]

Purpose

Join BrowserStacks open source sponsorship program, to enable automatic testing.

You are one step away from activating your subscription:

Please incorporate the text: 'This project is tested with BrowserStack' without quotes within the project's README.md file. Please merge this change into the main/ master branch at your earliest convenience to proceed.

Changes

Add text required to join BrowserStacks open source sponsorship program.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.