added stackblitz to playground (#2759)

* added stackblitz to playground

* modified link of Stackblitz

I changed the link based on comment from @EricSimons

Author: devpato

content/docs/getting-started.md

@@ -36,7 +36,7 @@ React has been designed from the start for gradual adoption, and **you can use a
 
 ### Online Playgrounds {#online-playgrounds}
 
-If you're interested in playing around with React, you can use an online code playground. Try a Hello World template on [CodePen](codepen://hello-world), [CodeSandbox](https://codesandbox.io/s/new), or [Glitch](https://glitch.com/edit/#!/remix/starter-react-template).
+If you're interested in playing around with React, you can use an online code playground. Try a Hello World template on [CodePen](codepen://hello-world), [CodeSandbox](https://codesandbox.io/s/new), [Glitch](https://glitch.com/edit/#!/remix/starter-react-template), or [Stackblitz](https://stackblitz.com/fork/react).
 
 If you prefer to use your own text editor, you can also [download this HTML file](https://raw.githubusercontent.com/reactjs/reactjs.org/master/static/html/single-file-example.html), edit it, and open it from the local filesystem in your browser. It does a slow runtime code transformation, so we'd only recommend using this for simple demos.