Add free React course by the University of Helsinki (#3789)

Full Stack Open course is part of the computer science studies in the University of Helsinki, and also offered as a free massive open online course (MOOC). The main focus is on building single page applications with React that use REST APIs built with Node.js. It is available in English, Spanish, Chinese and Finnish.

content/community/courses.md

@@ -24,6 +24,9 @@ permalink: community/courses.html
 
 - [Scrimba: Learn React for free](https://scrimba.com/g/glearnreact) - 48 hands-on video tutorials building react apps.
 
+- [University of Helsinki: Full Stack Open MOOC](https://fullstackopen.com/en/) - Learn to build web applications with React. Available in English, Spanish, Chinese and Finnish.
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 ## Paid Courses {#paid-courses}
 
 - [Egghead.io](https://egghead.io/browse/frameworks/react) - Short instructional videos on React and many other topics.