Add which-query extension to documentation (#569)

* Add which-query extension to documentation People come across the query page in the documentation, and they might not truly understand how to use `getByRole` or any other methods since they don't use to do that! They used to get an element by `class` name or `id` most of the time. Or they were simply using JQuery for many years. As a result, RTL's queries do not sound very familiar to them. The "which-query" chrome extension is a good place to help them get started. They can use it to learn how to access any element in the page, just by the help of RTL queries (instead of setting data-testid, ...) * link to Testing Playground extension

Author: m98

docs/guide-which-query.md

@@ -57,6 +57,11 @@ const { container } = render(<MyComponent />)
 const foo = container.querySelector('[data-foo="bar"]')
 ```
 
+## Chrome extension
+Do you still have problems knowing how to use Testing Library queries? 
+
+There is a very cool Chrome extension named [Testing Playground](https://chrome.google.com/webstore/detail/testing-playground/hejbmebodbijjdhflfknehhcgaklhano/related), and it helps you find the best queries to select elements. It allows you to inspect the element hierarchies in the Chrome Developer Tools, and provides you with suggestions on how to select them, while encouraging good testing practices.
+
 ## Playground
 
 If you want to get more familiar with these queries, you can try them out on